Wowee, this is a contentious issue, and not just in UK libraries. There was a great post the other week about the use of volunteers in archives: I recommend reading it, even if you're not au fait with the workings of the Society of American Archivists.
There's a real and unpleasant tension between the competitiveness of the job market necessitating everyone get experience even before they're employed in a library and the constant pressure to save money (in the short term, if you actually want to maintain a decent service I suspect) by losing staff. Of course, the latter also contributes to the former.
I've been remarkably lucky in not having had to weigh up my conscience against my need to gain experience. I did a few weeks voluntary work in my local public library when I was a third-year undergraduate, at the point at which I'd worked out I wanted to go into library work and had read the university careers service advice suggesting I get some experience. Even at that time (c. 2004/5) I felt a bit concerned that I was doing work, as a volunteer, that probably warranted 'proper' staff attention. I spent quite a while updating a very useful catalogue of the contents of song compilation books in the music library: vitally useful when someone's looking for a copy of a given song and doesn't know, or care, what book(s) it might be in. I think I did a good enough job of it, but I did feel that there really should have been enough paid staff time for it to be done regularly and not relegated to "when we have someone extra here".
I was considering offering up my services to various libraries if I'd not found a new post after my current contract ended. The areas in which I thought I could do with more experience (cataloguing in particular) lend themselves to volunteer help, I think, and there's always something to be catalogued somewhere. Other extra-curricular work, such as sitting on the Cambridge Library Group committee also counts as voluntary work, I suppose. (And the Group is always looking for new committee members, if anyone local is interested...) It's certainly a good way to develop new skills, and is a way to broaden your horizons without feeling that you're contributing to putting professional and para-professional out of their jobs.