I wrote this down from another thread on this topic about 12-18 months ago. I think it was in OW forum.

"Transit legs and FF numbers
Every system can handle different FF programs and numbers for the segments across a stopover, but maybe not for transit segments.
The transit capability depends on the computer used. Sabre can do it. Amadeus can't. CX's CUBE can't either. AA uses Sabre, QF/BA use Amadeus.

Even when the system can do it, some airlines/agents have trouble putting in separate numbers for transit legs (they don't know how to work the computer) so you may have to study up on the key sequences and prompt them if you really want to do this. But it works fine."

Kiwi Flyer's suggestion is good, because even if the change at LHR is accidentally made retrospective, you will have the correct number on both BPs to prove later what they should have been, and get a manual correction.