There's a hell of a difference between brake checking and slamming on brakes in front of someone, they way that chop did to me on Thursday.

( See Bad Driving Thread)
If someone is tailgating me, and I cannot move over, I put my hazards on to warn him, then I gently brake until he gets the message. It is totally beyond my comprehension that so many Cretins cannot see when a person in the right lane cannot pull over due to traffic conditions.
As far as the OP goes, if the Golf was sitting in the right lane at 120 then he was actually within his rights to stay there, however, before everyone gets hysterical, we do have a speed limit in this poor country, and the majority of the drivers out there think it relates to everyone except themselves.
If he's "blocking" the right lane at 120, then, unless you are trying to get to the hospital because your wife is having a baby, you should actually sit behind him at 120, and stop trying to break the law. If you genuinely need to get past, flash the guy a couple of times, and you will find that he will more often than not, pull over. Ducking past him on the left, then aggressively cutting him off is actually against the law.