Well, as a matter of fact, since the statute of limitations has now passed, I can confess that we sneaked into England that way back in fall 1974. My wife and I, with five kids and a babysitter, and an enormous pile of luggage, got off the QE 2 in Southhampton. I think we did show someone our passports on board.
Then the kids started to get antsy, so we proceeded down the gangplank, where we located our VW bus that had come over with us and had been lifted out of the hold by crane, and our enormous pile of luggage. I rented a small lorry for the drive to Cambridge.
Then we looked around for Customs. They had been absent all this while. It seemed they were having a tea break. So I loaded the trunks into the lorry, my wife got the kids into the car, and we just drove off. No luggage inspection. No questions asked. They must have been having a very nice tea break.
Just to see if it could be done I used to walk from adjacent postal facilities into the heart of the customs facility (in various port cities) and not once was I challenged.