Great Western Coffee Shop

Looking at Victoria in Australia most of the Melbourne area had over head wires by the 1920's ish
Melbourne to Traralgon main line was completed in 1954.
I think I am correct in saying U.K. main line electrifcation started in the mid 1970's
Should/could the U.K. have jumped from steam to over heads earlier?
Or was it a case of cost fuel being cheaper/political reasons?

The UK stated much earlier than that, the 3 constituent parts of Southern Railway had stated electrification before the grouping in 1923; the LNER had commenced 1500V DC OLE electrification of the routes out of Liverpool St before WW2, BR continued with 1500V DC post WW2 on the Woodhead Route.

BR took a brave decision in 1956 to adopt 25kV AC OLE electrification for the WCML and for future schemes, the WCML was completed to Glasgow in the early 1970's

  https://en.wikipedia.org/wiki/Railway_electrification_in_Great_Britain

The UK has been poor in its consistency with electrification which by and large is due to political decisions and not Railway Industry decisions

From 1913 former Clarence Railway lines were electrified with 1,500 Vdc overhead lines and electric locomotives hauled coal trains between Shildon and Erimus marshalling yard, which had opened in 1908 between Middlesbrough and Thornaby (Wikipedia)

From John Speller's Web Pages - Midland Railway (https://spellerweb.net/rhindex/UKRH/Midland/Morecambe.html)

The answer depends on how early "early" is, I think. In  "Outlines of Electrical Engineering" by Harold H Simmons* (Cassel, 1909), there is Section on electric traction, with three short chapters on trams and one on railways. He considers the economics of electric versus steam and (for trams) cable traction - but I can't see any mention of internal combustion, which seems odd (with hindsight, at least).

He concludes that electricity wins for trams, with the exception of very steep hills. Overhead feeding beats ground-level in streets, and always has done. For railways he finds the economics are not yet clearly in favour of electricity, but other factors (safety, smoke-free tunnels, and laws dictating the choice) often swing the choice in urban and some suburban systems.

For both trams and (sub)urban trains he sees the state of the art as mature - meaning good enough, not that it won't evolve incrementally. They use around 500-600 V DC, with an upper limit of 1200 V imposed by the commutators. AC motors were never used, since they had so little torque when starting and needing to accelerate.

Main lines, even then, were not suitable for third rail - a unanimous view, except in BR Southern Region down to quite recent times! While the Swiss had tried using AC motors, with a three-phase supply, this was a demonstration (in the Simplon Tunnel) and never found more general application. AC supply was being used, but with DC motors (which change direction by having the leads of their field coils swapped over). The commutators (again) and the inductance of the motors together meant that 50 Hz could not be used, but 25 Hz AC was already being used instead.

So while OLE was established, there was no clear way forward as to what flavour. With DC, voltages had to be too low to be efficient; while for AC, at 25 Hz the motors and transformers were big and performance not that great either. I think (though he doesn't say as much) he was not foreseeing big generators in public supplies, or a grid, but smaller steam power stations run by the railway. So steam locomotives still had a cost advantage against all that infrastructure and losses.

Again, it was external factors that pushed a few railways towards electric traction. Abundant hydro power, a shortage of coal, or long tunnels (all true for the Swiss, especially) are mentioned, plus imposed legal requirements again. He mentions the Midland Railway's line from Lancaster to Morecambe, but mainly as an example of the "Siemens" compound catenary (three wires). His main example is the New York, New Haven, and Hartford Railway, where the lines near New York City were forbidden to use steam. In various places they used third rail (bottom contact), an overhead rail (presumably also DC), and 11 kV overhead. The OLE was an odd system with two catenary wires side by side and V-shaped droppers to the contact wire - which seems now to be solving a non-problem.

*The full text (and some of the pictures) of Simmons was digitised by Google Books, but for some reason is only available from the Internet Archive (https://archive.org/details/outlineselectri00simmgoog/page/n830/mode/2up) - p787 (p832 on line) for the section on electric traction, and p815 (p860) for railways.

Didn't the Southern adopt third rail because the availability of their initially chosen OHLE system was interrupted by WWI?

On the evidence, it was more to do with standardisation.

The 1956 decision Graham referred to was contained in a BTC report called "Modernisation of British Railways (at the Railways Archive) (https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwiZ3N2HyJvuAhU6QkEAHWGEAHAQFjAAegQIAhAC&url=https%3A%2F%2Fwww.railwaysarchive.co.uk%2Fdocuments%2FBTC_Electrification1955.pdf&usg=AOvVaw2KOArZfT9FizpBlgX19Das). That is quite short, and includes a useful historical summary.

Before the 1956 decision, national policy was fixed in the 1932 "Standardisation of Electrification Order" (sadly not in RA). Generally, only 750 V DC third rail and 1500 V DC overhead were to be allowed. But the committee report the order was based on referred back to a similar one in 1927, and that only reviewed another such report that made the same formal recommendation in 1921 (but with no legal backing).

So the industry was already thinking along those lines before the order confirmed it, and through running was a major reason for all this standardisation. The 1921 report said that the LBSC should be allowed to extend their 6.6 kV OLE to Brighton, but grudgingly. The move to the "big four" in 1923 pushed compatibility across the SR up the agenda, so the 1927 report notes that the (by then) SR had decided to discontinue their existing system (and they had already done the extension with third rail).

It should be remembered that steam locomotives compared well to electric power in the early days.
A steam power station as used 100 years might only have an efficiency of 20% under ideal conditions and perhaps 15% in practice. As much as 50% could be lost between power station and railway line.
A steam locomotive might be 10% efficient under favourable conditions.

By the 1960s, steam locomotives had improved only moderatly, but modern steam power stations had reached about 34% efficieny and losses between power station and track had also been reduced.
The National Grid allowed the best power stations to run continualy at full load, with stations lower down the "merit order" being kept for peak loads and breakdowns. A lot of local generating capacity was scrapped once cheap power from the national grid was available. As a report said, "Even when local conditions require a non standard supply, it will be economic to take a bulk supply at high voltage from the grid, inccur the conversion losses, and scrap local power stations"
A lot of otherwise obsolete generating plant was reprieved by the war, and immediate post war shortages.

And prior to the national grid, most local power stations could not supply main line electric railways economically. Trams were bad enough.

Electrification was a higher priority in nations with no or little coal, but ample hydroelectric power. Here in the UK we had plenty of cheap coal.

I read somewhere that the lack of sufficient local power generation was one of the reasons that the IOW electrification in 1967 only electrified the railwy as far as Shanklin and the remaining few miles to Ventnor was closed.

Herbert Walker, the LSWR General manager, sent Herbert Jones, their Electrical Engineer, on  a fact finding visit to the USA in 1912. Jones was impressed by the USA 3rd rail DC system for its economy of installation and reliability in use. This led to its adoption by the LSWR. The scheme was presented to the Board in December 1912.
 (source Southern Electric by David Brown vol1 Capital Transport 2009)

I'm sure that's all correct, but those decisions are all after the end of WWI. I'm talking about something that happened immediately in 1914. What I remember reading is that the decision had been taken to electrify using overhead equipment and a supplier had been selected, based in Germany. Then with the outbreak of war, that became impossible, so they went for a British-made third rail system (possibly based on that already in used on the Underground?). However, I can't now find any reference to it, so possibly I'm getting several different things mixed up...

If you look at the sources I cited earlier, it's clear that the committees tolerated extending the "non-standard" LBSC system only because it was their standard. But they had only done their "suburban" lines - even if the choice of 6.6 kV was based on doing the Brighton main line. So the obvious conclusion is that what war stopped was the extension of their electrified network.

That's borne out by this page (https://spellerweb.net/rhindex/UKRH/OtherRailways/BrightonElec.html) - which adds that some small planned extensions were made even by SR after 1923, before they gave in to (presumably) industry pressure and the expected legislation in the 1932 order, as well as the logic of adopting LSWR's more extensive DC system as standard.

The demise of the London Brighton and South Coast electrification OLE system was brought about when Grouping happened, the London and Southwestern won the day in the board room of the Southern Railway they had a larger and more established third rail system.  There were also engineering problems with tunnels on the LBSC using an OLE system.  The LBSC 6.6kV was an auto transformer system which required an addition live conductor at the top of the support masts, also it was not 50 Hz which had become the UK standard ac power frequency.