You can use a Companion Voucher to for a one-way or return from the USA to anywhere, but it has to be on BA metal only.
SFO-LHR-HKG return would use 1 voucher.
However, you need availability for each flight segment. The avios required are the sum of each segment. SFO-LHR + LHR-HKG.
Ok - and the Companion Voucher is also known as a "241"?
If availability is what I get from their tool on the BAEC site, there's not much in terms of anything beyond LHR unless I want to visit Moscow.
But there is plenty to see in the UK and we could always jet around on RyanAir or something once we're there.