An interesting question here about how Silverlink could feasibly released 321s in 2003 to avoid Great Eastern, as it was then, having to take on 21 360s.Indeed. When the Class 360's were bought to replace the Class 312's, they weren't what FGE actually wanted - FGE wanted more Class 321's (and Eversholt were keen to provide) but Silverlink wouldn't release them and the nomadic 322's weren't enough on their own, so they had no choice but to buy new.
Eversholt eventually (years later) got their wish to consolidate all the 321/322 fleet in one place in East Anglia but sadly, politics and the ridiculous way franchise bidding meant rolling stock cost books get cooked, means that has now resulted in the entire fleet having no home...
Silverlink had 37 321s at the time but it was some while later before the 350/1 fleet was procured and even that was the by-product of a fairly speculative 5-car 450 order for South West Trains that had to be diverted when plans fell through.
If 25 321s had gone from Silverlink to Great Eastern to replace 312s, the problem suggested of non-standard 360s would have arisen at Silverlink along with a micro fleet of 12 321s.
Later, London Midland got 37 350/2s to replace the 37 321s which were meant to go to Great Northern, Greater Anglia and c2c. The c2c transfer didn't happen which resulted in 7 321s staying at London Midland but that was much later.
I guess the big question is whether, with 100% 321 operation on Greater Anglia, a full Renatus upgrade would have happened with stock replacement deferred for another ten years.