Leaving aside particular aircraft foibles, I am pretty certain that there is NO ATC requirement to 'nail' the speed immediately. I assume (ATC??) that the speed requested is what they want for the vectors they will give you, which will depend on many things, and that they would EXPECT you to achieve it in a reasonable time, which I always take as closing the throttles and starting the reduction when requested.
When you forget, speedbrakes CAN be used at Flaps 25 & 30. Just remember that they are not "recommended" because they WILL cause excessive deceleration and/or descent rates if held open too long...
- a note of caution for Intruder and others - use of speedbrake (in a 737) with flap above 5 will produce significant and possibly fatigue inducing tailplane buffet AND stall speeds are increased (10kts) with speedbrake, so my logic says that min flap speeds need to be thought about also. Whether this applies to the 747 I know not.