If you are planning to do CX to HKG and JL to TYO on the same booking be aware that it would price up as a multi-partner booking which may be more expensive in terms of avios compared to the normal per sector pricing. You can get around that by booking all on CX or booking the CX and JL flights separately- obviously some implications for connection protection if separate.

also when coming back, if you can book this as singles rather than as a return, a HKG-LHR booking would incurr very low taxes/fees/charges - around £30 from memory.