If you wish to trade mainly in the UK then I would use the co.uk if you are planning on trading internationally use the .com

I went for the .com with 105speed and although it gives a nice short address it does attract interest from all over the world, which would be fine if the people expressing the interest actually spent a second or two to read the first line of our home page to say we are UK based and then took a second to think what that means for shipping of goods and exchange rates etc.

Lots of people assume a company using a .com is in America or somehow not really tied to any particular country at all, its just out there.