This is usually an estimation if the customer has not recently purchased a vehicle, but you can use the millage and MOT checker to give an actuate estimate if the customer is unsure. Other sources such as Auto Trader and Gum tree are also useful.
Be clear with the customer though. There is no point in undervaluing a vehicle to get the price down. It is not always the case the cheaper the vehicle, the cheaper the insurance. Insurers sometimes look poorly against cheap vehicles as they view that the client less likely to care about there vehicle and perhaps drive the vehicle in a reckless manor.
Be sure to check and change the value range with the UW's as some insurers will charge differently around different thresholds. Use the below as a yard stick -
if the value is £1000.00 check if putting it down for £999.00 alters the price. Use this for all thousand multiples.