As prescribed in 229.402-70 (f), use the follow clause:
VALUE ADDED TAX EXCLUSION (UNITED KINGDOM) (DEC 2011)
The supplies or services identified in this contract are to be delivered at a price exclusive of value added tax under arrangements between the appropriate United States authorities and Her Majesty’s Revenue and Customs (HMRC Reference Notice 431, entitled Relief from Customs Duty and/or Value Added Tax on United States Government Expenditures in the United Kingdom). By executing this contract, the Contracting Officer certifies that these supplies or services are being purchased for United States Government official purposes only.
(End of clause)