So your biggest and most important form of marketing will be your passive marketing, and this will be the same wether you're targeting a UK market or a US market.
Focus on your cover, your title, and your keywords.
Are people actively searching for the book you have written?
You can find this out by going onto the US site and thinking about how you would personally search to find your book. Pay close attention to what is auto-populating in the search bar as you type.
If you're happy to share the genre of your book then I might be able to give some more help?
Rather than spending time developing a website right now, I would strongly recommend you bin that in favour of setting up a mailing list.
They are the single most important form of marketing for authors.
Is there any way you could write a small 5k - 10k prequel story? Could you take a side character from your novel and use him / her to develop an introduction into your world? Some piece of backstory that comes up that would make an interesting prequel and give readers a taste of what is to come?
If that's not possible then consider using the first three chapters of your novel.
Put it onto Bookfunnel, or I've heard StoryOrigins can be good too, and use the freebie (called a reader magnet) to entice readers onto your mailing list. They must sign up in order to get your free book. This will really help when you come to launch the book because you won't be starting from scratch. You'd also want a link in the front and the backmatter of your novel, again advertising the free novella.
All that said there are a couple of things you can do to subtly make the book more appealing to US readers. The first is pretty obvious - write in US English. UK readers don't mind this because we've grown up with knowing "toward" just means "towards", but US readers can be slightly more arsey about "typos".
The second thing is try to enlist a US beta reader. It's fairly easy to forget that subtle things might not make sense over there. Prime example - I once had a guy who pulled away from traffic lights on the amber. It wasn't even that important a detail - I was just showing he was impatient, but it confused the US readers because they don't have red > amber (get ready) > green (go).
But all that stuff is just sprinkles, really, and will only matter if you can nail your passive marketing well enough to actually attract the US readers to download / buy your book in the first place.
Best of luck with it and feel free to keep posting if you have more questions 