Thanks a lot for all your messages, will keep you posted. Will follow your cautious advice and take a c. 3 hours in between the two flights.
@toyotaboy95: yes I would have taken the direct option, especially given that HKG is not on the way, but have already booked the HKG-LHR return through BA sale
I think most of us here have been saying that even three hours may not be enough. There are ATC delays at PEK and there can be long immigration lines in HKG. Are you prepared to pay a walk-up fare for HKG-LHR?