Been visiting for a while and not posted but came across this and thought it was perfect for people who just need a small mobile allowance for an afternoon or day.
You can top up for:-
£2 for a days access or 500MB limit
£7.50 for 7 days access or 1GB limit
£15 for a months access or 3GB limit
Then theres also the possibility of Quidco - PAYG gives £3.50 and Mobile Broadband gives £40 - so Id assume it would be the former.
Its definitely available online but may also be available in store.http://shop.o2.co.uk/promo/o2mobilebroadband/tab/Pay_and_Go?cm_mmc_o=Kbl5kzYCjC4B_bkw%20PPCjCiCjCVyw5zY%204B_bkw%20__