You could still use the ferry service even if the airline is not on the ferry "check-in list".

The airlines eligible for ferry "flight check-in" is that you could already check in yourself for your planes and get your boarding pass there when you check in for your ferry in the counter, which also means the luggage is handled at the same time. the ferry service interlink them directly to the planes. You don't need to take care of the luggage anymore after the ferry counter.

And there are airlines that are not ferry " flight check in" eligible but ferry eligible. For these airlines (BA included), you simply buy the ferry ticket, including one free checked luggage per ticket, extra piece is like 30rmb/p. They would carry your luggage during the whole ferry. You are basically empty handed/with carry-on only after ferry check in counter. You then have the ferry boarding pass, get on board, enjoy yourself without luggage. And upon arrival in HKG, you pick up your luggages conveniently after deboarding, and go straight to the airline counters to check in for your flight. The 120HKD refund ticket per person is then given when you do airline check in there and getting your boarding pass. So anyone who takes the ferry to leave from HKG would be eligible for the refund, just that some get the refund ticket in ferry check-in counter because they do their flight check-in there, some get it only after they arrive in HKG ferry Port to do the flight check in.
All in all, ferry is the best in terms of the convenience and the luggage hassle free, comparing to other options. I would highly recommend it.

PS, you could use Ctrip.com to buy the ferry ticket. Just watch out the time of your flight. The ferry check in is closed 45mins before ferry departure. And ferry takes about 30mins to reach HKG