NATS cadets currently only get about £10k per annum in training, so it pays to have some savings tucked away beforehand/use your lump sum/persuade your parents to move to Bournemouth (delete as applicable) before starting out as a civvy controller.
You will get paid more eventually, but you still have to persuade a unit to take you on for OJT, if non-NATS, or take whatever job you are given if contracted to NATS.
You should also get paid for all the extra stuff we mil controllers are expected to do for free - unit trg team, UTO, LEO, watch roster writer.
And you will be able to join a very powerful union.
A few colleagues have gone across to civvy controlling and have loved it. Some experienced mil controllers, who we thought were good candidates, did not get through selection. Some have been binned off/withdrawn from civil training because they could not adapt to civil ways.