//£5,000 fine and invalidated insurance for having an unrestrained dog in your car.//
Please point me to the legislation that says, specifically, that you can be fined for having an unrestrained dog in a car. More than that, please explain why you believe a fine of £5,000 would be imposed.
I'll point you to the legislation that says it does not (indeed cannot) invalidate your Third Party insurance:
Road Traffic Act 1988, S148(2)(d):
http://www.legislation.gov.uk/ukpga/1988/52/section/148