I know he won't be as popular as the TV guests, but what about Todd Mccaffrey? His mother Anne (now sadly deceased) wrote the Dragonriders of Pern series of books, as well as many, many others such as the Crystalsingers and the Petaybee series. Todd is a writer himself, and has written some original works and is the recognised heir to the Dragonriders of Pern. He does do American conventions, but as far as I can see he hasn't done any UK ones yet. Perhaps he's never been invited?
Todd does live in America, but his family is based in Ireland, so you may be able to swing it! He would be a very popular guest with scifi and fantasy readers.
