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.)
Sorry, but according to news sources a coalition of non-US vessels are doing the bulk of patrols out there in the strait of Hormuz whilst the US is only seemingly wanting to lend 'command and control' style support.
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.
It's not about who patrols the Strait of Hormuz. It's about who and what backs them up. If the Iranians decide to sink the frigates there (to do their blockade) then they can expect a visit from a US carrier group. If there is no force that can be projected there then it becomes about who can move more stuff there. Seeing as that's literally next to Iran, Iran is going to win.
Look at Crimea if you want an example of what can happen if there's no force that can back you up.
The UK has a significantly smaller defence budget than the USA does. Which kinda proves my point.