The problem is that the primary radar coverage of the UK is particularly pants.
They have a whole heap of problems with a thing called mosaque which is the level which they swap over feeds from one radar head from the other. And there are huge areas that they can't get much at all.
And there isn't a mil topcover option anymore after they scrapped a certain airframe type.
So basically I can understand why the want things to have a transponder because otherwise they can't see alot of traffic.