Thanks to everyone for the helpful replies.
At this stage, we're evaluating the transfer offer and from responses, the option to decline is looking better... I've been told there could be a promotion in the next 6 months which should bolster my income by another 10k HKD/month, but the impression I'm getting is that even that would be insufficient. My initial thought had been that the salary would be enough for a similar transference in living standards (it's a decent salary even in London, although not outstanding), but that's looking less likely. I'll definitely chase the rental reimbursement option but considering it wasn't mentioned in the offer, I'm guessing it's not something the company will give. Also, we've been offered hotel accommodation during the quarantine period and a serviced apartment for a month to help us sort out living arrangements.
The 3 bed/700 sq ft standard came about because we were actually looking to upgrade to these specs in London. The extra bedroom would be for family we're expecting to visit (after covid). We'll pursue options in some of the areas mentioned over the next week as well. Discovery Bay was actually one of the first places recommended to us but a colleague told me the ferry commute will be a nightmare and cost a fortune. We'll look more carefully at that option as well.
Regarding the social unrest/political strife, we're fully aware and cautious of it. To be honest, we're considering HK as a stepping stone into Asia with the longer term plan to maybe moving to Korea later on when our child is nearer to school age. Compared to my perception of London (govt totally screwing up the covid response and message such that most people do anything to circumvent guidelines plus Brexit ruining trade and cutting off supply chains), I think a short term loss of some personal freedoms in a country I probably won't settle in long term is fine. Finally, the project pipeline in the London office looks a lot less appealing so this would be a positive career move... I've already been working with the HK office helping create a new team. There's less scope/appetite for that in the UK office.