That’s called brakechecking, a stupid and dangerous manoeuvre.
Most often used against lorry drivers, an even more stupid thing to do as they do not know the tolerance of the lorry driver concerned. Many years ago, long before the term was invented I was driving a lorry down the A1 in Yorkshire and a group of lads in a car though they would have a bit of fun. They overtook and brakechecked me several times. It is very annoying because we can’t easily get back up to speed, it means changing down, and I was driven to the limit. I had got to the point were I was determined that the next time would be the last and I would continue and ram them. They must have had a premonition because they then raced away and I saw no more of them. I shall never know whether I would have carried through with the action but there are plenty of examples on You Tube where drivers have been unable to avoid contact but, unlike me, they would not be prosecuted because of the greatest advance in motoring legality, dashcam. 
Just make sure the MOT person doesnt rip it off as it is in the swept area of the windscreen. (Ref to CritAir sticker)
I don’t think it is but in any case it is hidden from the inside by the sunblinds, which are always down. Not sure what the law is here about them but it has certainly passed several CTs over the years without comment. 