Metronet do an amazingly competitive pay as you go for £11.75 for the first 200mb. My aunt uses them and has had no problems. check them out at:-
http://www.metronet.co.ukmy isp Nildram (highly reccommended) has just launched it`s own PAYG starting at £15.99 for the first gig.
Personally i`m on their 1mb classic service at £34.99, though the 1mb PAYG looks good too.
http://www.nildram.co.uk