The speed limit all the way to SAM on the SAM SID is a real pain and to be honest I'm not sure why it's there. The majority of pilots ignore it and accelerate above 250 kts when they're above FL100. Which means when the odd pilot sticks to it you get caught out as they're inevitably caught from behind. If it looks like a pilot is sticking to 250 kts above FL100 I almost always cancel it and most of those who do stick to it usually ask if it's required.
I think the reason for it being there is to allow more SAM departures as they can be more tightly spaced in trail, however due to the sheer volume of traffic that flys through the bottleneck above Southampton chances are you will have received several vectors by the time you get there so the speed your flying is largely irrelevant.
But if in doubt, just ask.