Anonymous Coward may be overstating things slightly, but there is an element of truth in what they wrote. Any large business in the UK will register both .co.uk and the corresponding .com: they need both versions to prevent domain squatters muddying their name. So for example if you look at major UK supermarkets: tesco.co.uk, waitrose.co.uk, marksandspencer.co.uk all redirect to the corresponding .com address, Asda let you use either at the top level but all subsequent links are .com. Only Sainburys do it the other way round and redirect sainsburys.com to sainsburys.co.uk.
Smaller companies though may not want or be bothered about protecting their name that way so they pick one or the other. Also international companies like Amazon and Google often use the country specific domains to provide a localised service.