I hate to be a kill joy, but although Norwich will be requiring ATSAs over the next two years, there will be absolutely no problem at all in filling the vacancies locally, and I suspect that applies at the vast majority of UK airports.
Sadly, the previous training programme from atca to atco wasn't a huge success, and current policy is to employ fully qualified and experienced atcos directly - which still works well.

Upcoming ATCO retirements and a possible outsourcing of ATC to an independant provider will lead to atco vacancies, but it entirely depends on how the airport company or the new atc provider at the time wants to play it.