The ones I saw around London do stop you, either themselves or their colleagues further down the road. This is of course not to say that there aren't other setups where you get the letter in the post.
Regarding the latter, my understanding is that in order to do that they need more than a Laser gun, they actually need a system with a camera that can produce a photo, and these are typically located in vans and not carried by motorcycle cops.
What I would also say is that the fact that they they pointed the Laser gun at you isn't conclusive, i.e. they will often just be calibrating the device, or training a young officer how to use it. Additionally, in order for the reading to be valid they need to be sure it's you, i.e. if there are other cars near you or cutting in front of you then the reading won't be valid. And last, lighting conditions and reflection can also interfere with the reading.
In short, having a copper with a yellow hi-viz leaning on his bike and pointing a Laser gun at you is not a guarantee that you've been nicked.