IIRC three do the best offer - 10GB for 15 quid a month. it's a 1 month rolling contract. i can't check as the store is down.
i said 10GB, not 15. you'd have to be a lunatic to tie yourself into that sort of contract.
as the store is back up now, here is the deal i was talking about. 10GB, sim only, £15 a month....
http://store.three.co.uk/view/product/ql_catalog/threecatdevice/1021?tariff=3031