Rules is rules and what has been stated is quite correct.
BA was indeed asked to declare and transfer air miles to SBC. They gave a very firm 2-fingered salute. Remember not all air miles on your account came from Bessie. Imagine trying to sort out Mrs PNs air miles from mine.
The rules we have are actually identical to the USA as well.
They go on to say that you can accept an upgrade. Also you get to keep all denied boarding compensation or delayed meals allowances. If you do keep them or use them then you are not permitted to claim as well.
Clearly if the denied boarding compensation was less than the allowances (unlikely) then you hand over the DBC and claim the allowance.
The reason for BAs stance is simply commercial. If you know you get air miles then you are likely to manouevre your booking in their direction. Oddly only BA went to the places I had to go at the times I wanted to go.
When I had to fly Air France Club - brilliant. Lufthanza - le pits.