Thanks both for the replies. Not what was I hoping to hear, but thanks anyway
The LCY-GRX flight is a nice option, but unfortunately the timing of the flight from LCY is also too early to make on the same day as arriving from Beijing. I think we'll either have to take the BA flight to MAD at 16:15 (I guess if there are delays we can be moved to a later IB flight according to IRROPS procedures), or maybe we'll re-jig the plans to spend a couple of days in London on either side of the Spanish trip (rather than one block at the end of the trip).
Interesting also to read about the history of the BA/IB flight numbers - I hadn't noticed that it was the other way around on the MAD-LHR flights (maybe also the IB flights that don't fit the same pattern were added afterwards as extra slots became available?).
That doesn't look right to me either. I do recall that the fuel surcharge (YQ) on flights in CW for journeys starting in the USA can be eye-watering, but I don't think that applies to Y+ (the WTP cabin).
In any case I just ran a check on ITA Matrix for some dummy dates in August, and ORD-LHR(stopover)-KRK//WAW-LHR-ORD in Y+ comes up with taxes of around only 450 USD. In your case it should be a little higher than that as the LHR-KRK/WAW-LHR legs will be in CE so attract a a higher UK APD - but that is only 17USD extra (total very close the figure you were expecting). Using the Avios calculator tool on the BA site (pricing up as two separate trips and ignoring the stopover) also gives similar numbers.
I guess the open jaw at return is causing problems here. Maybe try pricing up a simple ORD-KRK-ORD trip to reassure yourself of the numbers, and then call to book the actual trip? Are you trying to make this booking as a Avios+GUF or cash+GUF booking?