Humbugs are really sweets, and they're available in most sweet shops (and lots of touristy shops) in Great Britain, although I suspect they are more a northern delicacy. Still, if you ask for humbugs people will definitely know what you're talking about.

Generally they're a peppermint hard candy a little bigger than your average Jolly Rancher, and they have a caramel or toffee centre. If you can lay your hands on some Irn Bru humbugs (they're yellow and triangular instead of striped rectangles) those are especially tasty :)