It can have SEO impact as Google primarily crawls from the US. They will from other locations but this has flip flopped over the years. This might be something that is causing this problem, if they are getting redirected to the US side but see many people mentioning the main domain (what you call the UK side). You can check by looking at your logs for when Google crawls and check the IP address in their list mentioned here. https://support.google.com/webmasters/answer/6144055?hl=en That will narrow down if it is a problem.
However, I do want to stress that using IP based redirects is frowned upon from a usability perspective. Giving people more choices is almost always better. A situation to consider outside of the issue that Google itself crawls from the US: When not in COVID times, people travel. I once tried to design my IKEA kitchen when traveling for work. I went back to continue my design I did one night in my hotel room, and it was gone! Little did I know, the design I did was on the UK side. People travel and you don't want to stop that from happening.
I get your point though, which is why it is preferred to do something like, funny enough they figured it out, IKEA does. If you go to ikea.com fresh, they welcome you with a message that they think you are in the US. It doesn't have to be a whole page, many sites use popups. But I would strongly urge you to consider this. And with that, I promise to drop the subject
Now, back to ranking. I see you at #6, and I am fresh eyes in the US space. I looked for NASA gifts and got this at exactly #6.
Where are you looking at ranking? Moz or Google Search Console or ?
I do see the issue with the meteorite jewelry. I see the URL https://www.thespacecollective.com/us/meteorite-jewellery?utm_source=georedirect -- It's canonicalized properly, so Google is being ... annoying. I would, if you have time, take a look at how that is exact URL is linked to. Somehow, that one is seen as particularly powerful, so if you can narrow down where it is and get that parameter removed, that should change over time.