Earlier quoted context omitted.
Anecdata, but since it was introduced, I've gone from getting a new bag every day (lunch/fresh groceries), to using the same one for pretty much the last 6 months. It was a very noticeable shift to making me think about their use. Buying one because I've forgotten is a rare, and slight annoyance (the price doesn't put me off, it just changes the mental model).
Lots of my friends have said the same thing. I've always wondered why people didn't re-use bags before the charge... Saving the environment didn't matter, but 5p does? People didn't know plastic was bad before? They didn't care? It's great that the 5p charge works, but do we need to have a 5p charge on every bit of plastic to drive the point home?
I'd argue "Yep". We've shown repeatedly we value convenience over the environment time and time again, I think we need to be pragmatic and, rather than guilt people into making better decisions, guide them to do it through this kind of (albeit fake) positive reinforcement - "save money by just using that thing you have".
Most people buying a loaf of Hovis and some baked beans of an evening give precisely zero sh*tes about the ecological concerns of their decisions. It'd be great if people just decided to be concerned, but I think we need to be realistic about the motivations of people, and their priorities.