In 2008, it was Mitch Benn.
Next year, it appears that not only we be getting Mitch again (and he's someone we'll go over to Norwich to pay to see), but also Ben Goldacre. Hooray.
It's not that people from outside both SF and fandom haven't been invited before, but I think we're beginning to see a new impetus to look outward more than has been common in the recent era.
(It'd be lovely to see Dara O'Briain coming along one day. Ah, I can dream.)
ETA: I should point out that the convention is at Heathrow - we drove over to Norwich just to go to one of Mitch's gigs, about three hours round trip driving.