Great Western Coffee Shop

Considering there size, and bodies could all 5 GWR super saloons run again between Paddington and Plymouth as a railtour set, similar to that of the Belmond British Pullman.

It would be a splendid sight  :)
But I fear that they would be ruled "out of gauge" despite having formerly worked over the route.
Also I suspect that they have dump toilets, soon to be banned, are vacuum braked, which is already banned and steam only heated, not certain what fuel is used for cooking but suspect that it might be LPG, which is either discouraged  or outright prohibited on the national network.

A nice idea though.

How would that work for Mark 1, Mark 2 coaching stock alongside the British Pullman fleet then

Whilst I stand to be corrected, I thought that all heritage stock used on the main line was air braked, or dual braked and used in air brake only mode. AFAIK, all steamers used on the main line are equipped with air brakes.
Dump toilets are still permitted at present, but I understand that network rail seek/hope/want to prohibit such toilets in the near future.
It would seem unwise to try and return to the main line any stock with dump toilets unless plans are well in hand for retention toilets, or for some  other work around such as locking the toilets out of use and providing retention toilets in adjacent vehicles.

Some steamers are vacuum brake only, also some coaching stock used on the mainline are vacuum braked only.
Fuel for cooking on some coaches is LPG.

The summer time Fort William to Mallaig Jacobite train is still vacuum braked and steam heated when needed.

I stand corrected in that case, though I still rather doubt that the stock in question will ever run on the GWR main line again.

Lack of air brakes on some steam locos used on railtours is why there is a diesel at the back to provide air and train heat although some sets have a generator van for power.