Section 1 was easy, Supernatural has a big fanbase and Misha Collins alone would sell loads of tickets, but I hear Jensen Ackles and Jared Padalecki charge a LOT of money.
I was unsure in section 2... I'm a big fan of both Star Wars and Lord of the Rings, but unless some big guests are announced, it wouldn't justify the expense for me. I voted for Life on Mars because, uh, John Simm. Need I say more?
Section 3... I love Firefly/Serenity, but most of the main cast have already been/will be in the UK attending other events (your Hallowhedon and others). I adore Glee, but is it really a convention kind of show? So I voted Merlin, because I've been wanting a convention for that show for quite a while, Colin Morgan, Bradley James, Katie McGrath and Angel Coulby are all lovely and you could probably manage to sign them all up. I don't think you should worry about ticket sales because there's definitely interest, judging by the fact that the fangirls have been invading all the children events organized recently for this show (the BFI screening, the talk at the Barbican) and the queue at Forbidden Planet for Bradley and Angel was as big as Torchwood ones (in fact I believe I saw many a Hub-goer in that queue...)
So, to sum up, the one I really really want and would definitely buy a ticket for come hell or high water is Merlin!
*crosses all fingers available*
Edited: Sorry, no mentioning other events on this forum. Thank you.