maybe individual gift vouchers...such as £10-20 each or whatver you choose, that way each person gets the reward they would like, some ... as someone else mentioned a joint meal for the entire staff would be very hard to plan and some may not really want to go on a massive works meal, some may miss it, some may feel left out etc...
you could always treat them with a selection of cakes in a hamper to share?
how about something like a new coffee machine for the staff room or something like that...?