The other night I popped into Sainsbury's on my way home and had a choc craving, but was loathe to spend a lot as I had a cupboard-full of the stuff at home. I spotted the Sainsbury's Basics choc and thought I'd give it a try; at only 23p for a 100g bar, it didn't really matter if it was horrible. As it turned out, I was very pleasantly surprised. It actually tasted really nice, and despite the low price, it was 30% cocoa solids - compare that to 20% for Dairy Milk (and I think 18% for Galaxy?). I'd really recommend giving it a try.
They also do a dark version, which I haven't tried as yet - but you have to splash out 28p for that one!