Thanks Steven. I basically tried to do just that although I didn't do the left indicator. However I think I'd have had to pull over and stop for it to have worked. Completely agree it's best to let them past so they can go off and have their accident with someone else. Also I don't engage with folk like this, don't attempt to speak to them or offer up a gesture of any sort. It can often just end up in tears - your own.
They are occasionally so unaware of their surroundings that indeed letting them go has them stop behind you instead...
Sometimes you can't fix stupid so you just need to try and protect yourself from it.
Others you let them past and then they end up stuck behind a car you were stuck behind..and you get to watch them be entirely enraged from a safe distance.
No point remonstrating they see nothing wrong with how they are behaving and are too stupid to see why it could cause anyone else an issue..