I suggest you do not buy a bow online at least if it is going to be your first bow. You are only setting yourself up for failure or get seriously hurt.
Force I suggest you visit a pro shop nearby so that they can properly measure your draw length which is extremely important.
I have a couple of good articles about this on my site.
Compound Bow Buying Guide - For Beginners
How To Determine & Measure Draw Length
Sadly I am in Canada so I don't know of any good shops in the UK but I would gladly help you with any questions you may have.