Regardless of any signs/markings being advisory, it would make sense that you use the road in the manner marked. A deliberate breach of that shows that you don’t care about the result of your breach. Thus they are in the wrong.
Both cars are in “ahead only” lanes, clearly marked as such. If they cut across your lane they’re asking for trouble if any other vehicles are close by. If you wanted to change lane or turn, you would need to wait or risk a collision. They risked a collision and got one.
If everything you’re telling us and showing us is correct, I’d pursue this to the bitter end. (County Court)
I had to do this where my insurer, Aviva, did ABSOLUTELY NOTHING to assist us (very loyal) customers. I then pursued the driver (NOT the insurer) making it very clear I’d not give up. He rear-ended and destroyed my car, and then admitted he nodded off at the wheel. His insurer, Saga, enjoyed taking the piss in endless phone calls and emails.
Their final email stated that I’ve ‘repeatedly threatened legal action’, without doing so. Guess what? Red rag to a bull. I won the case outright at County Court. Sleepy man got a CCJ against him, not Saga. He trusted Saga to fight his case, they didn’t even turn up.
Some people are very very stupid.
Make sure that your costs come within CC parameters though.