[Triband81]

You can get a prepaid phone bundled with a SIM in a wide variety of places in the UK. Virgin Mobile UK tends to be the best deal for roamers.

This question has been discussed below in the included links:

http://www.prepaidgsm.net/en/uk.html
http://www.prepaidgsm.net/forum/index.php?...323&hl=sim+card
http://www.prepaidgsm.net/forum/inde...topic=1301&hl=
http://www.prepaidgsm.net/forum/inde...wtopic=395&hl=

Furthermore, here are some links to prepaid packages in the UK:

Virgin Mobile:
http://www.virginmobile.com/vm/payAsYouGoPhones.do
http://www.virginmobile.com/vm/genericCont...rrif.shop.sm018 (UK usage prices)
http://www.virginmobile.com/vm/genericCont...uk.howdoi.sm032 (Calling abroad from the UK)


Carphone Warehouse:

Prepaid SIMs only: http://www.carphonewarehouse.com/commerce/...N&NODE_ID=10373

Prepaid Phone & SIM packages (all in one):
http://www.carphonewarehouse.com/commerce/...LD=MinimumPrice

Special offers:
http://www.carphonewarehouse.com/commerce/...B&NODE_ID=10372