Gatwick
Heathrow
East Midlands
Luton
Birmingham
Of the above, I would imagine you can only apply to East Mids, which is a non Nats unit, and BHX, which is soon to become a non Nats unit.
For the other ones I am not aware you can actually directly applyto. Nats recruitment is not unit specific, although in the past the company have done so causing the anger of employees and unions that must have been so furious to even declare themselves "peed off", all to be then swept under the carpet of the Working Together utopia.
However don't quote me on it - it wouldn't be the first time that an external candidate gets preferential treatment over controllers that have served years within Nats.