FF,

I know it's a bit of back-tracking but you can fly BA LHR-HKG Thurs morning, that gets you to HKG early pm. Spend shabbat in HKG, fly HKG-SIN on Sunday.

I always have similar issues with having to be in Taipei or Seoul or Tokyo for Monday morning client meetings, my default option is to spend shabbat in HKG. I have to do this even in winter as motzei-shabbat LHR-HKG doesn't connect to the last HKG-ICN/HKG-NRT on Sunday.