DD; I too tried to get posted from an 'ATCO 3' unit to an 'ATCO 2' unit for eight years without success, eventually I applied for a vacant post at my then present unit thus stayed an ATCO 2. Others were not so lucky; one who refused to accept a downgrade was compulsorily transferred to another unit, failed to validate and ended up at the college, so he won out in the salary scales in the end!!
I agree there should be a differential between units, but NOT as large a differential as presently exists, as it causes people (and we have one at the moment) to want to move on elsewhere as soon as they validate sorry achieve unit competency, thus there is always a heavy training workload and we're always short of dual valid controllers (ADI and APS)
By the way, I did the original ATCO cadet course; 3 years including PPL training with aerodrome, approach, approach radar, area and area radar ratings at the end. We also did a two week course with BEA (BA nowadays) learning how to fly a Trident (sims only of course) plus actual airways navigation at a flying school which taught CPL courses.