What a nightmare this is.
From "Bloke with an iPhone", to Seasoned Pro, the rates are almost always the key to getting the job on SME and below.
LondonFreelance.Org has a page dedicated to it at: www.londonfreelance.org/feesguide/index.php?section=Photography as does the NUJ and a myriad of other organisations.
Here's some rates from various photographers.
From these, and other sources, I've pitched myself at a "medium" level...
Well. Here goes. Nothing ventured, nothing gained.