It seems to me that, regardless of the "wishes" of NATS, if you quietly do some time at a non-NATS unit (within the legal restrictions imposed by SCATCROH, of course) then how are NATS going to know?
Even if they find out subsequently, they'll hardly sack a valid controller in these times of desperate shortage - will they? Just a rap across the knuckles and a "don't get caught again" lecture probably.
It is still your time off after all!!!