HKG has a main terminal and a satellite terminal, but for practical purposes only has one terminal. "T1" and "T2" refer to two separate landside check-in areas. There are no lettered gates at HKG.

CX and JL will both be from the main terminal - but it can be quite a walk from one end to the other. Although there is a transit train, which operates separately for the departures and arrivals level, between two points in the main terminal (and also the satellite terminal) you need to be familiar with the airport to know when it is better to take it or to walk.

If you are at the front of J/F, and you know the layout of the airport, and BA31 arrives on time, you can easily make it; but if you are at the back of Y, you are unfamiliar with the airport, or BA31 is late by 30 minutes (which often happens) you are quite unlikely to make it.