I think you will find most Porsche dealers are aware that a demo car qualifies for the first year rate,and are happy to supply a letter stating it has been used for demonstration purposes
CA23153 - Plant and Machinery Allowance (PMA): First Year Allowance (FYA): expenditure on cars with low carbon dioxide emissions
Businesses of all sizes can claim 100% FYAs on capital expenditure on a car provided that:
the car is ‘unused and not second hand’, and is first registered on or...