My plan is to travel from LHR to PEK, get an internal flight to HKG, and then from there back to LHR.

Now I hope to buy a BA club seat and upgrade to F to PEK, and then a 100%mileage J ticket with CX to LHR. These redemption seats are available.

My question is whether BA will charge me fully flex J price to PEK, or do they treat my return (mileage CX) as staying an overnight Saturday......

Hope this makes sense.