The domain extension of choice for businesses in the United Kingdom is .co.uk. It is what UK businesses use and has billions of pounds and over 15 years of cumulative marketing behind it. It is what UK consumers know, trust, and prefer. It means pounds, and not dollars. It uses distinctively British spellings, and not Americanisms.
Consumers in the UK have grown to distrust .com. After finding out time after time that the company on the .com doesn't sell their products or services in the United Kingdom, they have grown to trust and give a preference to .co.uk domains. In a 2010 survey, Nominet found that 80% of UK internet users stated they would choose a .uk website over a .com website in the search results. 77% of UK consumers stated that they prefer to shop at a .uk website over a .com website.
As well, Google.co.uk tends to rank .co.uk domains higher in their search engine results pages.
If you are serious about selling to consumers or businesses in the United Kingdom then your choice is clear - you've got to use a .co.uk domain for your website.