My recent experience, and a question for anybody familiar with British immigration authorities:
US to Spain and back, via Madrid Barajas airport (1996): Stamped in both directions.
US to UK and back, via Heathrow (1997): Stamped upon arrival, but not departure. Stated length of stay was 9 days, left on schedule.
US to UK, via Heathrow (2000): Stamped upon arrival. Stated length of stay was, IIRC, 2 weeks. Actually stayed almost a month. (I would have told the truth had I known what I was going to do, but my travel plans were somewhat fluid.)
UK to Ireland, by train: Not stamped, no forms to fill out, no questions asked.
Ireland to France, by ferry: Ditto.
France to Spain, by train: Ditto.
Spain to UK, by ferry: Stamped at both ends. Stated length of stay 8 days. Left on schedule.
UK to US, via Gatwick Airport: Not stamped.
Got all that? OK, here’s the problem: I have multiple stamps indicating entry to the UK and none at all indicating when (or if) I left the country. Obviously I must have left at some time or I wouldn’t have had to re-enter, but at least in theory, I could have spent the years from 1997 to 2000 bumming around the country and working illegally, returned to the US briefly, come back and done more of the same (with a short day trip to Spain). Is this the sort of thing that will raise any eyebrows on my next trip to England, which will be in December if I can scrape up the money? If so, can I do anything to document my good faith? Or am I just paranoid? (For personal and professional reasons, I anticipate having to go back fairly frequently; I have no intention of working illegally, but I fit the profile of someone who might – i.e., young and poor.)