And the UK can do that because the UK is willing to rely on the US. Now that relations with the US are straining in the West other western countries are increasing their military spending/cooperation. (Eg EU army.)
Elsewhere the UK assists the US in Syria and Afghanistan. It's not the other way around.
But feel free to tell me how great the american way of doing things is. What with fantastic provision of basic healthcare for its citizens, and sensible gun control oh yes and affordable education.
Look at Crimea if you want an example of what can happen if there's no force that can back you up.