Pay would not impact nationalisation, but Pullman was owned partly by the Southern Railway, and partly by Wagons-Lits, based in France. There were issues with nationalising ownership outside the UK. The same applied to the likes of the County Donegal Railway, owned half by the LMS and half by the Great Northern of Ireland, itself with ownership and operations in both countries.
There were some odd bits of nationalisation that may have not been intended - the Midland General / Notts and Derby / Mansfield District bus and trolleybus operation came in to state control through nationalisation of the electricity industry rather than transport.
My understanding is that the UK operations of Wagon-Lits, which included Thomas Cook, got taken over by the UK government in 1940 as it became 'enemy property' when France fell. TC then got sold to Hays Wharf Cartage, which again got nationalised when the railways (who owned HWC) were nationalised (have read things about this in various places, but also mentioned on
Wikipedia.
Presume by 1948, Wagon Lits was again in French ownership, but can't find specific references to that.
There were enough merger issues at nationalisation as it was. The same applied to the countrywide bus services that were nationalised at the same time. Some of these, as at Taunton, came originally from pioneer GWR road services, and the bus crews were still members of the NUR, not the TGWU as elsewhere, and this carried on into the 1970s (does it still apply?). Although they agreed common pay rates there was still some issue between the two - said to be the reason why there was no through local bus service along the Taunton to Exeter road, you had to change halfway and they were un-coordinated. Different onetime companies, different unions.
Yes, RMT do still represent bus drivers at a number of locations, and their
branch list includes much of what used to be the Western / Southern National territory, and mainly seems to be successors to ex Tillings operators elsewhere.
The situation has got muddied in some places with depots being closed, or sold from one operator to another, and some depots / operators have ended up with a mix of RMT (former NUR) and Unite (former TGWU) drivers.
I believe there may be one or two London bus garages (possibly relatively new operations that were never part of London Transport / London Buses) where drivers are RMT.