There was an interview on the James Bond DVD's with that one fellow who pops up in various 007 movies, which mentioned demand for American actors in the UK. (I think he also voiced characters on Thunderbirds.) I would think that made them a little scarce for tv shows, which probably didn't pay as well as movies.

There was one story on The Tomorrow People, (the original, not the remake) that was chock full of broad American stereotypes, including a not very flattering portrait of a certain president. Even as a teenager I felt a little insulted. Had the show aired in the U.S. in the 70's, there might have been sanctions imposed! ;)