How do I get across my passion for my subject without just saying how passionate I am?

You're already on the right track there - the one thing you don't want to do is use the word "passion"! When I was writing my personal statement, I remember being told that if a university saw a statement starting with "I am passionate about....." they wouldn't even bother to read the rest of the statement! The important thing is being able to PROVE that you are passionate about the subject - actions speak louder than words! Read as widely as you can (not just books relevant to your A-Level course), and if possible try to read a few books on the same topic area to show a particular area of interest (this will also give them something to ask you about if you are applying to a university that interviews, and you'll have more to say than just the contents of one book!). Also if possible try to attend events or talks that are relevant to your subject - for example, are there any conferences or university taster days that you could attend and write about in the statement?