I have been asked to reply on behalf of the Department for Foreign and Commonwealth Affairs.
No assessment has been made of the loss of revenue to the Exchequer as a consequence of NATO's tax-exempt status. This could only be done at disproportionate cost. NATO do not have a centralised accounting system or a central computerised search facility. Collating a reply would require coordinating inputs from around 40 NATO structures, located across all 28 NATO members states and tracking records back to 1957. Pre-1997 records are on paper files.