I think the Eurostar is the only port of exit in the U.K. where I _have_ had my passport checked - at least at my airport, exit passport checks are only random
I can't think of any other ways to leave the UK than by boat, train, and aeroplane, and I've been checked at all of them.
UKBF checks passports on departure at the Eurostar. At manchester airport at least, there are no routine outbound passport checks by UKBF
On travelling to the continent by boat (hook of holland) checks at the far end. Eurostar checks at re near end. Flying checks at the far end.