I have used the speedbrake before and slowed to 250 knots at the SLP just before being asked to fly 300 knots. Asking first makes it more comfortable and efficient and helps your situational awareness.
Area controllers are not permitted to tell you to disregard the SLP's in the London TMA unless they obtain prior permission or have you on a faster speed for their own separation (usually streaming). So when given initial descent asking "any speed for us" will not give you a response to keep the speed up all the way in.
BOAC - to answer b) then in Brownstars example being told standard speeds you can accelerate to whatever speed you want and then slow back down to 250kts at either FL100/SLP whichever is appropriate.