The value of a shipment as declared on your customs invoice/declaration is used to determine any import fees that may be applicable.
Understating the value to reduce import fees is an offense and can result in penalties.
Please be advised that the UK are actively targeting undervalued shipments into the UK and are taking the hard approach of blocking the shipper account permanently, stopping the customs clearance and returning the shipments back to the origin.
Please note that undervaluation is a serious offence under UK law and can result in fines and other penalties.
Yours sincerely,
Andrew Busuttil