As Grannyg and KittyLester said, if you have a decent John Lewis near you, look at their choc and sweets dept?!
We visited Cambridge the other week (fantastic free museum!) and in the town centre JL store were delighted to find a wonderful Qual Street pick & mix stand, if you like the green and purple ones (etc) just fill your boots (well, bag, box or plastic tin).
As for the "chocolates are smaller" moan, yes, they have been for many years now! A Mars Bar used to be a massive thing, now it's more like a match-stick, and KitKats haven't tasted right for 20 years or so??? I think it's something to do with the price of cocoa, at a guess . . . .maybe with global warming we could grow it here in future . . . .