Now, you don't get given a bag for your Sandwich & Water at lunch, and you don't get a free bag with your McDonalds meal to name just a couple of services.
1p or 5p wouldn't make much of a difference? A Family paying 100 pound a week on shopping at a super market, can easily just spend 100.35 on 7 bags and won't bat an eye lid.
If that was 107.00 that would make a real difference and I bet the decrease would be much greater.
The same acts in reverse.
I personally reuse the same bag every time I go shopping until it is damaged. When I walk to my local, I carry the two or three items I bought directly, rather than in a bag. I could afford to buy over a million plastic bags a year, yet I've changed my behaviour to avoid buying them whenever possible.
(In my case, it has nothing to do with being asked, as I invariably use the automated checkout machines. Bags are available for purchase adjacent to these machines, so it's not an availability problem either.)