I'd definitely recommend it for work experience - and it's usually good fun as well! I've volunteered in a couple of Oxfam shops on and off over a few years and really enjoyed it. It was also really useful when I came to applying for jobs as it gave me something to talk about in interviews - charity shops are often run by just a small team of people so you'll probably get the chance to do lots of different tasks, rather than just being stuck behind the till the whole time like you might if you worked in a high street store. They usually seem to prefer you to work for about 2 hours at a time, but only really expect you in once a week so it's not really very much at all (of course, you can do more if you want!) and the regular commitment will be something employers appreciate.
Good luck!
Jenn xx