Lakeland has them http://www.lakeland.co.uk/Homepage.action too, but I think larger supermarkets (not the little "Express" or "Metro" ones) would probably have them too, and possibly the likes of John Lewis.

For best result you need to have had the freeze packs frozen overnight and kept in a freezer until shortly before you fly. If you can't, at least keep them in the fridge or a chiller bag all day. If you are thinking of packing them without the chiller bag, it will not work properly on LHR-HKG length of flight - freeze packs do not stay frozen for long outside a chiller bag. If you cannot do this, I think you should forget the food that needs to be refrigerated in case they go off.