I ordered and just received a SIM from Asda to use for 1 month while I am here. I had previously intended to opt for a £10 bundle of 1.5 GB and 600 mins plus texts, however I just noticed they have added a 30-day Xmas bundle with 20 GB and unlimited minutes/texts. Yes it's only 3G but still a bargain as a temporary solution and much more than they usually offer. Hope someone finds it useful.
Despite what it says on some pages at the site you need to text "MEGA" and not "MEGA 10".