As I understand if you are travelling through LHR, staying airside in your stop over, you do not need to comply with the UK entry requirements but you still have to comply with the Dutch requirements and as such, as the declaration include all the country you have visited in the last 10-14 days, the UK would have to be listed and thus the testing requirements now apply.

edit: assuming my understanding is not spot on and <12 hours/airside only stop over does not require testing, I think the airline restriction implementation may not allow for exception as it would have been set on the side of cautions as in case the Dutch won’t allow passengers in they would be required to fly you back and potentially also pay penalties.. hence BA let you fill and sign those stupid disclaimers once on board for all EU destinations.