It's London - you'll find everything from £100 (and less) to £1K an hour
If you're only advertising on AW then it should be possible to find an average for your specific location through the search facility, but as ever if those are the rates you're comfortable with and get good business from then go with that.
If you're turning so much business away it's ridiculous then you can put them up, and if you're quiet at that rate then put them down. Or put up some more ads