I’m going to type this without really thinking about it too much, but British sitcoms feature cameos from stars of other shows much, much less than do American sitcoms. Here’s Jonathan Frakes making a slimy appearance in Wings.
And here he is on Cybill, sending himself up.
This is from 3rd Rock From The Sun, where someone appears to have stolen his beard.
And finally, he gets first billing in this scene from Futurama.