Courtesy of "Im_just_careful" (MSE).
iPhone 4 Micro SIM - I guess can be used with an adaptor in any phone.
SIM card cost: £5
Internet - 12 months free internet worth £40 when bought with £10 top-up
Quidco £5 apparently.
Not good for calls: just 100 minutes for £10 monthly topup; 20p/min otherwise.
Usual T-mobile Data fair use policy
You can browse and email as much as you want.
There is a 500MB allowance for streaming videos and uploading and downloading files. If you hit this limit during the month, you won't be able to do this stuff again until the next month.
£10 IF PURCHASED INSTORE
They seem to have changed the address. The new one is http://www.t-mobile.co.uk/shop/iphone/micro-sim/
Also, I don't remember this: "Please note that the 12 months free internet is for the iPhone 4 only and is not available for the iPad or any other device." I think they can't know what device you are using though.