ATSA's becoming ATCO's
The best route to take is through the non-NATS regional airports that run a full ATC service.
Several have a very good track record at taking on ATSA's and then putting them through ATCO courses, but you can expect to be 'bonded' to the airport - i.e. forced to sign a contract making you stay at that airport once trained, so they get their money out of you in the long run.
Put in speculative CV's and sell your aviation background, which will give you much more of a head start than someone with no aviation experience at all.
If you can't get a job as an ATSA then consider training as an Aerodrome Flight Information Service Officer. Again jobs can be as a rare as 'rocking horse *****' but go down the speculative CV route to the appropriate airfields and you never know what could come of it.
Britkit