It doesn't quite seem fair does it?
Assuming your domestic legs are not in a qualifying booking class themselves however, it is the way BA intend for it to be.
Another way of looking at it is that you've got the Scotland-LHR return for approx GBP90.00 (or less) above the fare ex-LHR (assuming this is paid and not a redemption), and a domestic GBP90 fare wouldn't get seat selection either.
I agree it seems picky.