Actually it's nothing in UK is cheap now as the Pound so expensive! I just back from the UK and Spain, everything in London so expensive including Boots and Bodyshop, it's not worth to bring things from there with the same price (or even more expensive) with HK!
But bookshops is a good place to shop! They have many books (both for your kids and you) that you can't find in HK, and they're cheaper than HK even in Pounds! Better go and check!