I can't provide you with a complete answer but there are numerous employers of ATC staff. Many Local authorities employ their own staff at places like Luton, East Midlands, etc., and there are a number of companies supplying ATC staff elsewhere - SERCO, HIAL come to mind.
I have worked for one of the "supplying" organisations, for a private company providing ATC at its own airfield and for NATS. Thirty years ago I would not have hesitated if asked who was best to work for but I'm old and approaching retirement so my opinion hardly counts! Hopefully some of the younger people will express helpful opinions.
Good luck whatever..