As snigs says, the 250 hr deal is for specific contracts that run from 3 bases around the UK, the 400 rule is accross the board pax and freight. I must admit, generally both the 250 and 400 hr guys we take are normally well known to us, having done their IR's with BFC, who are part of the same group.
Thats it really, the minimum requirement for us to operate is 400 hrs, I'd like to say it's because our training is better, but in reality, its a loop hole in the whole JAR / CAP thing.