I am probably BU.
I just think that M&S charging for plastic bags for food (not clothes) is utterly ridiculous. I usually use my bag for life but occasionally need to run in for some items on my way home from somewhere. On these occasions I need a plastic bag (which I would use again) and resent paying for it. If I was buying a pair of socks on the other side of the store I would get a free one but for food I don't. And don't get me started on how much packaging they use themselves, so it's hardly a green thing.
I realise there are more important things to get annoyed about but AIBU to resent paying for plastic bags.