sure sim by cable and wireless cheapest-a fiver,

from their site-
New SIM Pack: £5 including £5 of calls

Peak* Off peak*
Sure mobiles (Jersey, Guernsey and Isle of Man) 12p 12p
Local and UK land lines
(Jersey, Guernsey and Isle of Man) 15p 12p
Non Sure mobiles
(Jersey, Guernsey and Isle of Man) 20p 15p
UK (mobiles) 35p 35p
Europe (land lines) 35p 35p
Europe (mobiles) 35p 35p
US and Canada (mobile and land lines) 35p 35p
Australia, Bermuda, Hong Kong, Japan,
New Zealand, S Africa and Singapore
(mobile and land lines) 40p 40p
Rest of the world (mobile and land lines) 75p 75p
Local texts (excluding premium rate text) 8p 7p
Texts off-island (excluding premium rate) 10p 10p
Picture messaging (Sure mobiles) 25p 25p
Picture messaging (other mobiles) 30p 30p

Minimum call charge – 10 seconds on all price plans.
Other chargesFree servicesWeekend calls and texts (limited promotion ends March 08) Free**
Call display Free
Voicemail retrieval (local) Free
Call waiting and call divert (local) Free
Emergency call Free

Mobile Internet Local internet usage (per kb) £0.001p

*Peak time 7am – 7pm

**Subject to "free weekend" promotional terms. Requires top up of at least £5 once a week.