If you are unsure of the value of your vehicle you can try a valuation site such as www.parkers.co.uk. Do bear in mind that the value you place on your car should be the amount it will cost to replace it with another of the same make, model, age and condition.
The value you state may not be the amount you will receive following a claim as it will depend on the time of loss and the assessors valuation when the loss occurs