From Heritage: Speaking at the same event, Dr. Liam Fox, the British Shadow Secretary of State for Defense, explained why the Treaties are in the interest of both nations. For the U.K., the treaties offer economies of procurement scale, i.e. the ability to buy in the larger U.S. market. For the U.S., the treaties offer the possibility of more exports, plus the more valuable benefit of cementing the U.S.-U.K. alliance and insuring interoperability with our most important ally.
More fundamentally, for the Conservative Party, Dr. Fox noted, shifting procurement from European to U.S. sources was a matter of strategic importance: more cooperation on defense procurement within the EU for Britain means a weaker Britain and a weaker NATO, because so few of the EU nations are willing to fight.