Moral: If you are or ever intend to sell to both the US and UK markets then you should consider buying BOTH a .co.uk and a .com domain name. It will be much cheaper to do this now rather than later when you realise that your second choice domain name is already taken and to retake control your brand name you will need to pay an inflated rate!
Tip: if you have more than one domain name you still only need the one website! You can configure both domain names to point at the same web site, this is called 'web forwarding' and is just one of the FREE services that you can take advantage of when you register domain names via 123-domain-reg.co.uk!