Fundamentally Andy, yes. I would suggest a read of the two HC rules I pointed out.
The point that's trying to be made, and irrespective of right or wrong as that's been clearly established, is that upon noticing the left indicator you should have taken deliberate steps to pull ahead in order to minimise the chance of being punted off the motorway. Fortunately, he looked before moving over, otherwise you would be both squished and correct.
In order to drive defensively you need to anticipate other peoples 'misjudgements' and be prepared to mitigate them.
In that situation, I would have accelerated slightly to get ahead of a potential problem and possibly been prepared to head towards Hull if the situation had escalated.
If that's not something you were prepared to do, then perhaps you need to consider why not... If the positions had been reversed but same vehicles, what would you have done? Be honest with yourself as you answer that...
If someone else choses to be an arse that's on them, but if the response is to also be an arse because you are technically correct, then that's all on you. Sorry, but that's how it is.