Don't get caught up in things like probability of validating. I'd suggest spending your energy on working hard, not annoying your colleagues at a unit and taking heed of what they have to say. They've done it for a long time. Seems like an obvious comment, but one which is sadly ignored by some people training. The minority I hasten to add.
But, in answer to your actual question... I can't really give you an answer. Validation numbers will vary wherever you go. Area units I believe have a lower percentage making it through to validation, although I stand to be corrected. It's obviously a harder job...
In all honesty, it just varies unit to unit.
Just work hard at the college, keep your head down, get to a unit, work harder and enjoy the fruits of your labour. It's a top job and NATS is a pretty good company to work for depsite some grumbles you may come across.
Good luck.