Not sure how long your flight connection between your JPN-HKG, HKG-LHR flights
If the connection time is under 24 hours, you can check-thru the bag all othe way to LHR. (Generally, CX can check thru bags on two CX-CX tickets.)
During your connection time in HK, you can enter HK, and also check-in another bag.

(If for some reason, the CX ground staff can't check you thru, you can pick up your bag and check in your bag at departure)