I feel like the UK not having a huge military and the world's reserve currency as their own isn't as big of a deal in comparison to some of the other issues that the UK has. They tried to be, as you said, US-lite, but the UK doesn't have the massive domestic market, the abundant natural resources, and the population that the US has.

The US can constantly flirt with the idea of isolationism because its large domestic market and access to resources allows it to do so with potentially less harmful side effects than a small island nation like the UK that has always sustained itself off of international trade. This just plays into the idea that a lot of people from the UK view themselves as an island in the middle of the Atlantic rather than just off the coast of Europe.