^^^ You obviously shop in different shops (or possibly a different space-time dimension) to me, Hopkirk.
The websites for Asda, Tesco, Sainsbury's and Morrison's are currently all showing £3.50 as their price for boxes of 12 'original' Felix packs and £3.75 for the 'As Good As It Gets' type. The prices in Waitrose are £3.50 and £4.60, respectively.
Just don't expect it to be cheaper on Amazon. The boxes of 12...normally £3-4...are about 12.
I bought a box of 40 Senior several months ago for £11...over 20 now.