Both my son and I have been astonished, when on our various trips we passed the customs booth at Gatwick... and there was no one there. Texas has some very nasty cattle diseases that one would think the Brits would just as soon not have in their country. When I go back to the States, US customs asks very detailed questions about whether or not you've been walking around in farm country, and on occasions they make you sterilize your shoes.
UK customs did frisk my son and open his luggage when he came back from St. Malo on the ferry. He had mentioned he liked it in England and would like to live there someday, so they were searching for a resume. They were very, very concerned.