I saw and ad on the London tube about Lycamobile. I haven't tried myself as yet but thought to post it as it sounds as a very good deal.
It looks as it uses Orange network: http://www.lycamobile.com/PDF/Orange%20is%20seeking.pdf

With Lycamobile you can call the world for Less.



Lycamobile to Lycamobile UK Call 0p*
Lycamobile to Lycamobile SMS 7p

National Landline 8p
National Mobile 10p

SMS National 9p
SMS International 10p

Calls to Spanish,Italian, Australian, French for example are 4p a minute.

You can also keep your number. All information is available at http://www.lycamobile.co.uk.

Shops that sells these sim cards are all over the uk. I think you have to buy a £5 sim. Anyway on ebay they are also selling some for £2.50.

Once you register they give you free £2 credit.

If you top up £20 you get an extra 50% (so you will get £30 for £20). I think you get some extra credit also if you top up less (not sure how much)

www.lycamobile.co.uk