Try HobbyCraft, the big arts and crafts store? I know theres a big one in Reading, and I'm sure I've been to one up in Nottingham.
You can buy one inch circular magnets there (and in various other sizes), which I find fit perfectly on the back of the pans. You can also buy the strips of magnetic tape, which you can cut into squares that fit.
Failing that, a lot of other craft stores sell similar things.
Hope this helps, a little.