BA's practice is standard worldwide. The flight was overbooked and at risk of oversale. No reason to offload people until it is oversold.
By standard, I mean at the airport. As BA was covering hotels and the like, it is a simple matter of not making other plans which can't be cancelled until after the oversale is confirmed.
As a passenger this is a reverse auction. You are by no means required to accept it and could certainly tell staff that you will only do so if staff will process your rerouting and various other goodies on the spot.
EUR 250 seems low for a 2-day voluntary delay, but there are apparently people who will take the offer. No reason for BA to pay out more when it need not. In addition, if there are no takers and the flight winds up with an oversale, BA will simply deny boarding to someone and that person will be entitled to the EUR 250 and a duty of care until BA can get him home.