Orange:

http://freesim.orange.co.uk/6?utm_source=Google&utm_medium=PPC&sissr=1

O2:

http://freesim.o2.co.uk/

Three

https://www.three.co.uk/Pay_As_You_Go/Free_SIM/Order_a_free_SIM?id=1184

T-Mobile

http://www.t-mobile.co.uk/shop/free-pay-as-you-go-sim-cards/?WT.mc_id=ON_QM_S_Google&WT.srch=1

Virgin

http://www.virginmobile.com/vm/simCardOnly.do

http://www.freesims.org.uk/ lists all free sims and how many you get!

Well that's obviously not true, because we've already found an offer that they haven't!

And, I think you'd be silly to ask for more than you need and are going to use yourself. Networks must keep a tally of how many sent to a specific address, and maybe log how many of those actually get activated. So when you go back for another one, or someone else a your address does, the request will be ignored.

Can't recommend Giffgaff highly enough..

Giffgaff.com