If they don't have calibrated speed cameras they can pull you over but can't prosecute ( even if marked car) - no proof.
Of course if they pulled you they would say it was dangerous driving (which is worse potentially).
Of course they can still attempt to prosecute you for speeding, the officers opinion is enough for that. You either take the fixed penalty or argue it in court and local magistrates tend to believe what police officers tell them unless you can provide clear evidence you weren't speeding.