CharlesRhona the scenario which causes this is outlined in detail a few posts above by continentalclub.
This is due to the constraints of the online booking system, NOT because any seats are being “blocked”.
Doing research online using one way dummy redemptions, writing down all your required flight information is the way to go. Then call up BAEC directly with this information.
You can book exactly the same flights using your 2for1 as with the normal mileage redemption procedure; there is absolutely no constraint to availability using a 2for1 vs. regular mileage. None.
You just might have to call up to access it, that’s all.