Let me put a couple more points which may help address why the Tarka^▸ (Line from Barnstaple to Exeter) line trains are going to be 158s.

GWR^▸ (Great Western Railway) has 43 carriages of class 158 in the franchise and the Cardiff - Portsmouth service of hourly trains has an 8 hour cycle.  At 3 cars per train, that was 24 carriages spoken for ... and two more for the Brightons and you've spoken for 30 carriages ... couple of other trains running with 158s (remember there are 13 x 3 car and 2 x 2 car) and by the time you take a couple of sets out at a time (overhaul etc)  you've used the full fleet.

Increase the 8 trains in the cycle to 5 carriages, and immediately you've used 40 out of 43.  Even if you send something else to Brighton and on rigorously remove the extra diagrams, you ain't got enough.  So how about bringing some more into the franchise?    You know how much we like the 158s - well, tough, so does everyone else and the cries of "you ain't having mine" ring out from Thurso to Norwich and Fishguard ...

What else, then?   There are 46 carriages of class 150 in the franchise - 20 x 2 car plus 2 x 3 car. I suppose all the Cardiff - Portsmouths could be 158 + 150, but the 150 units are top speed 75 mph not 90 mph, so you'll effect performance, and the 150 carriages will probably be less popular - and they're also mighty useful on stop / start lines where lots of people are getting in and out every few minutes - Devon Metro except for Barnstaple, Falmouth, St. Ives spring to mind, as well as Bristol suburbs / Bristol Metro.

You can't couple up a 2 car 165 to a 3 car 158 ... so there's another possibility gone.

Castle and Classics cannot run via Hamble, so they's out

Courgettes probably aren't cleared via Hamble either, and running new 125 mph trains on a service that's not pointing towards London would be infra dig anywhere south of the Watford Gap.

So, though there are questions on their suitability, the Cardiff to Portsmouth run's got the trains cascades from London to Reading electrification - the hand-me-downs, the rescues.  We do love our rescue dog, so perhaps we will learn to love them.   Even though better suited to shorter runs and, to be fair, lots of journeys on Cardiff - Portsmouth ARE shorter ones.  Wide mid-carriage doors help the people who pile in and out at intermediate stations  - Temple Meads, Bath Spa, Westbury, Salisbury, Southampton.

So - where do the 158s go?   They're ideal for Cardiff - Taunton - Beyond.   They're ideal for the Cornish main line, and the regional extensions of that.  They're also ideal for the Barnstaple line, as the majority of its traffic is end to end - you don't have major flows of people on and off at Morchard Road, or crowds at Copplestone so although some services stop a lot, there's no need for provision for heavy intermediate loading and unloading.

The point about gauging of the 165 / 166 has also been made ... and there is sense in keeping trains that can be coupled to each other on the same set of lines, keeping maintenance of particular types in particular depots.

I can see why there are 9 carriages of 158 (out of 43) on the Barny and, yes, I would love to see them routinely on the Weymouth run - which DOES have the metrics for which they are very suitable and 3+2 is not (long, long journeys by many people to / from Weymouth).

Long post ... some application of guesswork and logic therein, but as you shake out the figures you start to see "why". Please don't shoot the messenger - the Cardiff / Portsmouth should be running with a fleet of 12 x 4 car 172s - but that order was cancelled in favour of GWR electrification, which has brought us the turbos.