When someone swerves in from lane three on the motorway and then immediately flashes you to move over…

What’s the best way to non-verbally communicate back to them that you had just started to overtake the car that’s been going at 45 in the left lane for several miles and as soon as you did, they sped up to match your speed so now you’re either going to have to slow your speed enough to drop back behind them with a safe braking distance (so they’ll have to back off your tail enough for you to do that) or gather enough speed to complete the overtake and therefore will be in the way in the second lane for a few seconds longer?

Of course that was the first opportunity I had to overtake them because until that point the middle lane was chock full 🙃

I’m so annoyed by lane hoggers and tbh I’m mostly just miffed that I have now been assumed to be one of them when I always do my best not to be haha