Following on from what unwise said... In my experience (just 31+ years in NATS and its various predecessors) controllers don't get chopped unless there is a very good reason. Considerable time, money and effort is expended in training, both at the college and at operational units. At all units, trainees will be with some of the world's most experienced and respected ATC staff and if, after all that effort, somebody is not up to the job then they will have to go.. You might be amazed at how many people THINK they are God's gift to ATC but whose Training Officers dread to sit with them - seen it, done it, got the tee-shirt and thankfully said "good riddance" to a few!! Some people who do well at the college can't hack live training - some don't even get past initial simulator work in the training section. Others have personality clashes, "personal" problems, etc., but every effort is made to help these controllers. After all, an extra bloke on the watch means more summer leave all round.
However, trainees with problems are usually given several chances (as some people reading this can surely confirm). Failure to validate at a particular unit usually results in an offer of a posting elsewhere. At a big unit the opportunity may arise to train on a different sector, which often works. I saw plenty of really first class controllers who had experienced great difficulty on previous units/sectors. Excellent Tower controllers have failed on radar... superb approach controllers couldn't do TMA to save themselves, etc, etc. If somebody is "removed" from NATS they need to think seriously about themselves because there must have been a good reason.
OK... now for the smart remarks...