So i went to do an exchange at primark, i had a £1 of credit left, so thought i would get a sim card .
I was suprised to find out that they were only 40p, they had all the usual carriers available.
This was the Tooting branch. Hope it is national.
Yes i know you can order sim cards for 1p or free, but you have to wait for them to be delivered, if you want a cheap sim like i did and want to be able to use it instantly then this might help.