PM
Too slow with side slip!
The gliders (sailplanes) that I have side slipped loose the ASI once the side slip has developed due to the airflow around the pitot.
I have also tried to stall a glider whilst in full side slip...never achieved it. I presumed because of partial blanking of the elevator.
As to airbrakes or speed brakes;
I used the latter term when I returned to gliders after years of big jets...had no end of criticism and wasn't allowed solo until I used them like airbrakes to control the rate if descent rather than speed brakes to control the speed.
The reason the BGA is so strict is because the number of undershoot accidents with pilots putting the nose on where they want to land and in a progressive undershoot case they can end up stalling trying to reach the threshold.
When I stopped instructing the current philosophy was to teach a half to two thirds airbrake approach...any less close them completely until you reached the "slot" ...any more pick another aiming point.