Under British Law a shotgun must have a barrel length of no less than 24 inches. If it does it then comes under the classification of a Firearm . These require different Licenses to possess them and firearms [rifles and magazine shotguns that hold more that 3 rounds ]have to be specified before purchase . It would mean in effect you would need to get a specific license for the Clarke to be shortened ,then have it transferred from its current licence and back again when the work was completed .