Even though it is 90mins, I suggest it is better to leave it over 2 hours.
LHR is the worst European airport ever!!! It is always over crowded and the security check is a mess!! Plus, CX flights sometimes delay from HKG to LHR. Or, it is also possible that air traffic of LHR is too crowded and the flight couldnt land on time.

Anyway, when transitting via LHR, always prepare plan B if you lose the connecting flight...