Complex sentence: The Treasury agreed to seven prototypes (two Hawker, two Boulton Paul, two Fairey and one Armstrong Whitworth) but only prototypes of the P.82 and Hawker were built.
Split into simpler sentences: The Treasury agreed to seven prototypes (two Hawker, two Boulton Paul, two Fairey and one Armstrong Whitworth). Nonetheless, only prototypes of the P.82 and Hawker were built.