As noted above by other posters, there are sometimes exit passport checks (by the passport people, I mean) on leaving the UK. They were abolished (around 2000, I think) but then the government had a change of heart and announced that they were to be introduced again. So far the re-introduction has been been halting and sporadic, and it may have stumbled a bit recently. Sometimes I am checked (most commonly at Heathrow, but not always even there), but often I am not. As far as I can see it is fairly random as to whether the passport desks are staffed or not.