I don't have any problem with that either. If you want to double or triple your price because .uk is included then you're free to do so. But at least tell people from the outset or offer it then from the outset at a fair price. Don't keep quite, let them build their .co.uk business up and then come out of the woodwork a few years later.
But that you are talking about is selling pairs of domains, co.uk + .uk end users have told me when offering them that they are not interested as they do not see the value, but I can.. so in time they come back and say.. ok well we must have it now, so what then say ok here you are free of charge. Business cannot run that way.