Virgin mobile are doing a great range of sim only tariffs at present. All are 30 day contracts so you can cancel whenever you like.
The one which might be of interest for members here is the Premiere Sim only contract for £20 which gives 2500 min, unlimited texts and data, plus the added incentive of unlimited landline calls which will come in useful for some of the services offered by members here. You can also add blackberry services for free on top if you need them.
More details here
http://www.virginmobile.com/vm/paymonthlySimOnly.doI can't see any mention of a FUP.