Keith. Thanks for "Heads Up" on this. So far I've only seen it stocked by Firstcall (expensive there). I use the online ordering system at Jessops occasionally. I'll check to see if it's available there. If it's in your local store it might be an over the counter, stock item held by all branches. Logically it should be.
Have you not tried 7dayshop.com. Its only £2.20 a roll for both 120 and 35mm.