What should I consider when buying a laptop from Hong Kong to use in the UK?

You shouldn't even need a travel adaptor as Hong Kong and the UK use alike species of power cover http://electricaloutlet.org/type-g If you use a modem (I know, "how quaint") they are compatible too. The lone problem is going to be the warranty if anything goes wrong. You'll be covered simply if it's "return to base" repair that could be difficult.