Right, there's a bit of nonsense being said in this thread, so let's try and clear it up a bit, shall we? This is all for England and Wales, I know Scotland is a little different.
1. You do not have to be stopped and issued with anything. If you did, then speed cameras wouldn't be acceptable.
2. They do not need to video you or take a photo or anything. All that is needed is a prior opinion of a single office of the law of excess speed. They don't have to measure anything, just that they believe you to be travelling above the speed limit. That's it.
3. Anything over 100mph is a court appearance. Not true, an officer always has discretion, and they can take you to court for 1mph over the limit if they wish. In practice, there are bands and you will usually end up with an FPN below certain speeds, but this is in their discretion.
If he took your plate, he can go back to the station and fill the paperwork in. You'll then get a S.172 in the post just like you would if you got done via camera. You'd then have to nominate yourself and go down that whole route. This is very unlikely to happen, so as I said just take it as a warning and keep the speed down, you never know who is behind you.