Firstly congratulations on your new venture of starting a domain Reselling site. It’s a good idea. Also since you are in IT and if you are doing web development, I suggest apart from domain Reselling you can also provide web hosting services.
Also answering your question that many domain resellers are based in USA and whether this really matters, well it doesn’t really matter in domain reselling as long as they offer the tld and service you need. But if your customers want .UK domain , if you can find a domain Reselling provider within your region it can be beneficial for local SEO purposes.
But if your also hoping to provide web hosting service. Then yes the location matters. The physical distance between your customers website’s visitors and the server hosting your website can affect the loading speed of your site. If your customers target audience are based in uk choosing a hosting provider with servers in or near that region can help reduce latency and improve the overall speed*of*the*website