Commuting back to the UK? Dont do it... PS you dont get to choose between 320 and 330. We have had type rated guys turn up with oodles of 320 time and put on the 330. Depends what the company needs to fill. Same pay though, and similar rosters. Neither of the rosters are commutable, although a few try. They usually end up angry, and try and buck the system. Would be much easier to commute from the desert than from here.
20k wont get you a livable 3 bedroom pad anywhere in HK.
Contact the schools for debentures and more importantly place availability. You wont be spoilt for choice I am afraid.