there are companies which purchase goods in USA on your behalf and send them on to UK, or other countries; they are known as re - shipping agents
The best known is (logically enough)
www.reship.com though there are others; search t'Net
You identify what you want, inform the agent, who then purchases & forwards it. there is a fee of course, but I don't think it's too bad
another approach would be to have something made to measure. check your area for people who do tailoring, dress alterations and so on
www.pennineoutdoor.co.uk supply fabric and other materiel for making your own gear. They have been established for years, and could probably pass you on to someone who can do this
www.highandmighty.co.uk supply men's clothing in tall and large sizes; mostly fashion/everyday clothes, but if you don't ask, you don't get