Not entirely sure about this, but a couple of thoughts. First, I can't see why not - they are entirely your own original creation, therefore you would own the copyright and, for example, if you couldn't do it then newspapers wouldn't have cartoonists.
Second thought is that one thing to be careful of would be a defamatory depiction of a famous person.
Final thought would be that there is a slow encroachment at the moment towards the development of the concept of image rights within UK law - and image rights do actually exist in other jurisdictions - therefore if, for example you were going to do one of Wayne Rooney you might find lawyers chasing you for some kind of licence fee.
Hope that helps