Why every major navy studies hypersonic missiles

Are Britain and France working on hypersonic missiles?

Probably, like every major power, because developing a countermeasure requires understanding the weapon whether or not the program is publicly acknowledged.

Major powers are likely to investigate hypersonic weapons even when they do not announce the work. A navy cannot design effective detection, interception, or operational responses without understanding how the threat functions. Research therefore serves an offensive and a defensive purpose. Countermeasure development provides a strong reason for apparently quiet missile programmes.