I wonder how much of that is to do with the applicants? As soon as I joined I had to take an Approach Radar course which, by coincidence, was running at the same time as the Primary course which I would have been on had I not explained that I held a valid UK ATC Licence. Several people on that Primary course had licences but had simply done as they had been told and not questioned the decisions.
I left Heathrow Tower for TC in 2002 and there were certainly several non-NATS controllers who went straight to Heathrow from UK airfields, and they are still there.