Why are fees so high for flying through LHR?

For business class tickets (including award travel) departing out of LHR, passengers are subject to the Air Passenger Duty which applies to all airlines.

Specific to flights operated by British Airways, the carrier imposes charges which are much higher than other airlines. These charges are more apparent for award travel, and less noticed on revenue fares. When redeeming for award travel, the carrier imposed charges are passed to the passenger which devaluates most redemptions flying with British Airways.

For award travel, search for flights operated by Finnair (via HEL) or Iberia (via MAD or BCN) to avoid these fees.