Don't know if this has already been posted but I did a search and couldn't find anything.
Argos are selling 1GB pay as you go iPad sims at the moment for £4.99 (instead of £9.99) which is cheaper than a top up for an existing sim!
I popped into a 3 store and asked the guy when the sim becomes active and it doesn't become active until it is actually inserted into the iPad!
This means you can pick up a few at a time for this super low price and keep them until they are needed!
I myself will be picking up a few today ;-)
Hope this tip comes in handy! :-)