Hi there.
Go higher! If you go too low it doesn't do anyone any favours - if you undercut all the established freelancers it creates friction and also you won't earn the going rate. Putting up rates with an existing client is never easy and you don't want to be stuck on a low rate!
I've been freelancing for almost 10 years now. My starting rate in London and the South East was £12 through an agency in 1999. I now charge a minimum of £20 through a agency and £25+ if going direct. I believe agencies charge between £25-£30 and you get around 80% of that.
I have heard that rates may be sliding a little due to the current climate, but not by more that a pound or two!
Hope this helps!