Unfortunately in the event of an insurance claim, they usually go by Parker's or Glass's guide as to the price of your car. It wouldn't matter whether you had it insured at 18K or 180K, if it's only worth 8K, in the eyes of Parker's, that's all they'll pay out, after a struggle.
If it's the other way around they don't bother... As in if you insure your car as worth only 4K, but Parker's says £6K, they will only pay out 4K as that's what you said it was worth.