Im not entriely sure, but it seams when i go to google.com and search for an item, i will get american results ie- everything will be in dollars, it will give me the american Ebay and Amazon... So personally if im looking for a UK company, i use google.co.uk to search.
I guess it would make a difference to appearing higher up if you are UK and im only searching for websites in the UK - it would remove all the searches in america?
technically it makes no difference at all if you use a .com, .co.uk or any other TLD (top level domain)
From a marketing standpoint then a .co.uk will make UK based people feel more comfortable that they are dealing with a UK based company (though there is nothing to stop somebody anywhere in the world using a .co.uk)
most companies I've dealt with would register both the .co.uk and the .com TLD if both are available and then simply point them both to the same website