You’re being screwed, NIG can’t understand the situation as it’s clear that the other driver has “Failed to Give Way” the fact that you’re passing the traffic on the left is totally irrelevant. It is a straight forward Right of Way Violation. I don’t suppose the police were called?
Remember the Insurer doesn’t represent you… and based on my personal experience NIG are lazy! Tell them that it’s 100% not your fault and that if they settle you will take them to the Insurance Ombudsman.
Get it all in writing asap.