Embarkation checks were abandoned at Channel ports by the Tories in the mid 1990's. When Labour took over, they used this as a precedent to abandon embarkation checks at the airports. Now no-one really knows how many visitors, students, work permit holders etc actually leave the UK when their visa expires - an awful lot simply stay put and enter the world of illegal working, sham marriages & bogus college courses. Of course, if you are unfortunate enough to be picked up by a UKBA Arrest Team, then a timely claim for asylum will get you instant bail and another chance to disappear. Until embarkation checks are re-introduced, the UK only really operates half an immigration control. Last time I visited the US I was checked by immigration on departure and I'll bet they haven't abandoned their outward checks.