Prop suppliers
Buying props
It is a good idea to buy props well in advance. If you wait until the day before to buy that vital UV pen, you will find that every branch of Ryman's and Maplin has run out of them.
UK web retailers
London prop suppliers (bricks and mortar)
Costumes
Good comprehensive range to buy or rent: Angels Fancy Dress, 119 Shaftesbury Avenue, WC2H 8AE http://www.fancydress.com/content/london_store (as of June 2009)
Steampunk/vintage costumes: Prangsta, 304 New Cross Rd, SE14 6AF http://www.prangsta.co.uk (trading as of June 2009)
Toys
Hamleys is worth wandering round for inspiration until you get a headache. There's an amazing number of cheap toys which suggest gimmicks for games. The staff are not very good at knowing what is actually on sale outside of their little department.
Chocolate coins
Hamleys has these as of June 2009
Anything vaguely technical
Maplins is a good bet