I support the charge but I am guilty of not taking spare bags with me very often.
Years ago, shops did not routinely provide bags. I remember when supermarkets provided strong paper carrier bags but they charged for them.
Freee plastic bags create a demand for a product that is made from a bi product of oil (a precious commodity), that is non-biodegradable and which uses up a lot of energy in production and distribution.
However, 5p is rather a lot for the standard of M&S bags - I find M&S bags split really easily.