I am currently with Chitter chatter. They basically sell you a T-mobile contract and give you cashback every month. SIM-only here: http://www.chitterchatter.co.uk/simonly.php
They have changed their layout so it is harder to understand now, but you get what is stated in the table, plus 1 free "booster" (e.g. "unlimited" internet or landline calls - there are others as well). The amounts in the table are after cashback. Everything has been fine for me: T-mobile have the best signal in the areas I use my phone, especially as you can also use Orange signal (but no 3G when on Orange - i.e. slow data speed).
Note: T-mobile now seem to charge a premium for accessing your voicemail on SIM-only plans (which I find annoying and sneaky), but you might be able to avoid that if you ask about it when signing up. Business plans also do not have this charge it I believe.
So yh, I thought I should mention it here as it is not very well advertised. I thought it was amazing when I first saw it. However, a friend got a much better deal on Orange when she tried to switch (from Orange), although that may have relied on the fact that she had been paying far too much for a long time before that. On the other hand, perhaps ringing up phone companies (in general) can get you better deals than these. Nevertheless, I am using the £6/month deal with an iPhone 4, as I just need the unlimited internet really.
I don't work for these people, but I do like to verify a good deal. Feel free to let me know of any other SIM-only deals that you have heard (I am free to switch at any time really). E.g. some amazing deal that you got which is not advertised.