I am aiming to start emailing people in various physics departments in the UK asking about PhD's, but I am really not sure what I am supposed to say in this first email. I mean, I don't think that "Hi, I want to do this PhD please. Bye" is going to do me much good.
How long does it have to be? Am I supposed to go on about how much I want to get it, how much I have researched about it and all that business, or just keep it short and sweet?
Thanks.

Hello, when I was going through this I generally sent a sort of short cover letter, sort of an application form, then attached my CV. It was a couple of paragraphs long. Check the Uni website before, see if there are any funded positions etc al;ready available so that you show you checked what they can offer etc