If its a brand new service shell and you have receipts to prove then its just classed as a replacement part so you do not require a SABTVRO DVLA issued VIN number. One of those is only required when its a previously used shell being built up into a car with no donor. In such cases you also have the option of compiling your own unique 17 digit VIN number.

Dealers bodyshops swap out damaged shells around the country daily for insurance companies and have been doing since the car was invented, and they all retain the original cars VIN. Joe Bloggs has as much legal right to do the same thing as a dealer.