Sounds like a good idea for a topic, maybe you could expand the idea to how the influence of British law and our style of justice system influences those of other countries and use the former colonies as example? Of course it depends on how much resources you have to write for 5000 words.
From a person who wants to do Law at uni next year also, doing an EPQ based around law might help a little bit in your application, at least that's what my EPQ supervisor says. Btw, where might you want to go to do Law?
I'm doing a law-based project as well, my idea for a topic is whether law should be separate from morals and ethics. It wasn't my first choice but we have to include morals and ethics for some reason.