I regard the booking system as one of BA's great mysteries.
Our usual booking is JER-LGW, then LHR-IAD, with an MFU from WT+ to CW. So, you can't do that on-line, because ba.com can't do multi-city with MFU. OK ... we phone, having first checked the price of the LHR-IAD on a dummy booking.
The final bill is usually within £1-£5 of the dummy booking for the LHR-IAD ... and the JER-LGW is usually booked into CE [40TP each way] at effectively zero cost. I think our last booking cost us £1 each for JER-LGW.
I guess it largely depends on who the BAEC agent is, and which buttons they press, to cope with an MFU from ET to CW on the LGW-JER.