The simple reason is, the british stopped being a big empire in the wave of decolonisation. They lost most of their colonies. Not just for military reasons, but human progress. Self determination of people etc. So had no need or funds to sustain a mighty global fleet anymore.

(And health care sounds like a way more useful thing to fund, than the capability to wage war around the globe)

Maybe the US needs to learn that lesson, too? Right now all the US with all its might achieved is blocking a former free flowing transit.