Ask if you can have onion ring crisps instead. Or chocolate. Or Cake.I have just looked on M & S site and they don't sell knickers for £4, the cheapest single ones are £6 as are the cheap multi packs.
Looked at the T and C's and you can only have 1 pair that cost 3 or 4 £'s so this is an absolutely useless offer just like the Percy Pig cushions that were not available. I can feel another email going to M & S customer services.