As everyone has said above. Essentially, if the Police vehicle is on the other side of the motorway, then there's not a lot they can do other than radio to another car that might be down the road on your side. Even then, they need to have a reading on their camera in the car which shows you over the limit for a set distance. So you're okay there.
Most of time, when the Police are sat in those emergency bays, they're doing ANPR checks rather than looking for speed related offences. Unless they obviously see someone hoon it past of course.