I'm 18 months into a move from London to Hong Kong.
Good things about Hong Kong:
Everything is close by, feels safe, more of a 24/7 culture (though the centre of the big UK cities are still quite lively at night), shorter commute for most people, public transport is cheap and reliable, cheap to eat out, taxes are much lower, air con in the summer, helpers for childcare and domestic duties, less drunks
Good things about London
More space (though much more in the regions), air is cleaner (just), general public more considerate (e.g. holding doors open rather than letting them go in your face), heating in the winter, easier to buy high quality fresh food (e.g. organic meat) to cook at home, education system more balanced (but getting your children into the better schools can be almost as hard as it is here), more multi-cultural, more cultural options for theatre, concerts, shows etc.