Learned a quick lesson today.
Regarding BA award travel with AS Miles. 70,000 miles
Had 2X - 3/21
BCN - LHR Business
LHR/- SEA FIRST
Today, decided to try for a layover in London. Found award travel again on BA. (BA seems to only have F award seats on Thursdays)
Changed to
BCN/LHR 3/21
3/28
LHR - DEN FIRST/ DEN-SEA Main
Fees went from $310.31 per passenger to $540.31 per passenger
AS award desk had no idea why it increased. Same itinerary travel on same day fees were the $310.31
As he has me on hold, I found the answer here.
Stopping in the UK for longer than 24 hours will trigger Air Passenger Duty. In this case, it was the $200 or so increase in fees.

Thanks Calculon, good info. Even AS does not seem to know this rule. When he came back with still no clue, I gave him the answer and he said he would pass it on to the others.