Shouldn't be a problem buying Windows 10 from the US over the Internet. Plenty of US companies are willing to ship stuff to the UK, some even offer free postage. It doesn't matter where you buy since you can customise the installation for any country, so you don't end up with a dollar or hash symbol every time you hit the pound key.
It isn't practical or reasonable to restrict the operating system to work in only the country where it was purchased. If that were the case, there would be no point in having portable computers. What does tend to happen is that the initial set up might assume you are using a US layout keyboard if you are using a US sourced copy of W10. You just select the right keyboard layout and country later if necessary once the software has been installed.