When you sign up for Bluehost you choose between 'I need a domain and 'I have a domain', choose the latter to transfer your existing .co.uk domain.

Bluehost is a hosting service. It just happens to offer a free domain with its hosting plans! So, you'll have to register your .com through a domain name registrar, like http://www.123-reg.co.uk, and then transfer your domain again from within your Bluehost control panel.

Bluehost support is notoriously good. You can talk this through with a representative via live chat on the Bluehost home page.