oldart
In theory they could operate as a Customs Warehouse.
in which case they could import and store goods duty and VAT free until removal. If removed for export to outside the UK duty and VAT wouldnt be paid. On removals to the UK the duty and VAT is paid.
IP (inward processing) or End Use traders are the real catch.
The advantages of a free port already exist in single applications for the likes of CW, IP and End use.
All free ports do is bring those advantages into a defined zone, on a simplistic level.