Worth remembering that should you train at Gatwick and fail to validate, ANS have only Edinburgh to possibly post you too. There are only so many ATCO’s needed there however. At least one such failure was offered a Lighting Operator position at Gatwick, but long term this role will be automated.
NATS, having more airfields potentially, may be able to potentially offer those failing to validate at busy airfields more sedat3 locations.
There are other ANSP’s however to apply to if you fail. Few of these would be of interest perhaps to either ANS or the greater NATS I think.