This is one of the situations where they have you over a barrel. You had to make your Eurotunnel crossing, so you couldn't hang around. That was unfortunate. I would usually reject a car with an actual crack (rather than just scratching), as hitting a pot hole could further the damage. You'll often need to be persistent in declining an assigned vehicle.
Also, the contracts are usually worded so they can pretty much charge your credit card for any reason. People need to be more aware of the risks.
I've also had terrible service at LGW. It sounds like not much has improved. I hope this bad PR gets your issue resolved. These are unfortunately not once-off incidents.