All numbers GBP- my pound key won't work
Re HK, we did (14 years) but not everyone does. It depends on your salary/ package and your lifestyle when you're there and also your "benchmark"- i.e. what you were spending in UK. Income tax in HK is essentially 15% flat (bit more complicated than that but good enough for planning purposes). It's becoming less common to get a full package to move to HK, especially in very high paying jobs (front office banking/ partner in law firm) as employers consider the tax break sufficient to more than compensate you for your inconvenience
. Probably fair enough
As you rightly say, cost of living and in particular rents are very high. Again these vary a lot- you can spend 25k a month on rent for some chichi apartment on Middle Gap road or you could live in a spacious village house on an outlying island and spend 3k. The expat average is probably around 5-6k on rent per month but huge variations. The other thing is school fees- unless you put your DC in at kindy, it's very difficult to integrate them into the (free) local system, so you are looking at 18-30k per child per year in school fees, depending on school. For many expats, they were already paying or planning to pay for private school in UK so it's just the same, but if you've been using the state system in the UK, or were planning to, then it's an additional cost.
Things that are cheaper: cars, transport in general and (big one) childcare- the minimum wage for a full time helper is about 650 per month - now most of them won't work for that but it would be rare to pay above about 1000 per month . However, the childcare cost and coverage (6 days pw live in) does make it much easier to sustain two "big" jobs with the requisite travel so you can max your earning potential.
Arguably, on a basic salary, rent + school fees - tax is a wash from an income point of view vs UK, However, where people save a lot is only paying 15% on their bonuses/ share issues which are often multiples of their basic. Honestly, I think for HK, this is where people who save a lot of money save it - getting that additional 25% of your bonus (15% tax vs 40%) AND being able to sustain two big careers - I know anecdote isn't data but from observation there were far more "2 big earner" couples (as in 150k plus basic plus big bonus) than in UK.
However, you are probably right that at lower levels of law, the bonus won't be a large enough component of your total comp to outweigh the extra COL.