I've been to a few LotR conventions and I've not yet seen JRD or Andy Serkis. I met David Wenham in Milton Keynes last year and found him to be, as I gathered autographs for several friends, unfailingly courteous and charming and good fun, but I'd love to have the time to ask him questions and have the opportunity to have a photograph taken with him.
I'm a bit puzzzled by the "I've already met him so wouldn't bother buying a ticket to see him again" school of thought. To me, it's a bit like saying, "No, I'm not going to see Manchester United play again: I've already been to one of their matches." I've heard John Noble speak four times and there are still questions that I want to ask him that I've heard no one else ask him. Yes, you might have heard many of the LotR stories but the actors and people from WETA are not one-trick ponies: they have done things before and after LotR. I am going to another show in Germany BECAUSE I've met many of the actors before and I know them to be good value.
RingConUK is very expensive and it has a long, long way to go before it becomes good value for money. I sincerely hope it succeeds, but I'm not very hopeful.