Just to clarify. There is nothing defective regarding the Harrier GR.1. It functions exactly as it did, doing all the same things it was sold to you doing as advertised. BRs of all vehicles are subject to change and this is always the case for all vehicles premium or not. They have not, nor have they ever been advertised or claimed to be a fixed system. They are a dynamic moving system that moves with the performance of the vehicle. All BRs are subject to change and we always publish those changes within the patch notes whenever we do them as we did today with the Harrier. Nowhere was it ever stated the BR cannot change and you did not pay for a product that was promised or advertised this would not happen. Therefor nothing is defective, breached or wrong. Dynamic Battle Ratings have been a part of the game since Update 1.37 in 2014.
The Harrier is not a defective product, nor did you buy it with any promise or advertisement that its BR would not change.
I would also point to you the legal End User Licence Agreement and Terms of Service for the game. Both of which you agree to have read and understood both when joining the game and when making purchases: