Dumb question and off-topic but is new rolling stock decided by the TOCs or the DfT? I assume it's done based on the age of the stock and if it's cheaper to upgrade than tinkering.
AIUI the norm was the DfT would include some specifications for rolling stock in the franchise specifications but the bidder would also have some flexibility in exactly what rolling stock they leased. Sometimes the government would gaurantee rolling stock leases beyond the term of the current franchise, other times the rolling stock owners would have to fend for themselves in finding a new home for their rolling stock.
There were some high profile exceptions though, the IET project was a government scheme, i'm not sure exactly what the arrangements were for the Thameslink and Crossrail rolling stock.