Wouldn't a better idea be to divide them along subclass lines. Since the complaints seem to suggest the seating and layout is more based around being a 'metro' type service couldn't they leave the 700/0s (8-car) as they are since they'll mainly be operating the shorter more 'metro' and stopping services anyway from what I can gather. They could then upgrade the seating and layout on the 700/1s (12-car) since they'll be mostly operating the faster and longer distance services, have 4 more coaches anyway and although I don't know for sure (someone will probably tell me) might not be quite as crowded as the shorter distance stoppers anyway.
Cambridge to Brighton is hardly a metro type service.