Well, but if Spain is the destination, passing up UK makes sense. On the other hand, APD leads to calculations such as
Flying NYC-LON-Continental Europe: No problem; as long as your time in UK is less than 24 hours, you are a transit passenger and don't pay APD.
Flying Continential Europe-LON-NYC: Not so good; you pay the APD on the trans-Atlantic leg, and that's high.
The other issue with this is not APD at all; flying to the Continent through Madrid or Barcelona tends to be cheaper because it usually doesn't involve BA, which has the highest "fuel surcharges" of any of the Oneworld partners.