Been a long while since i've booked an agency girl, but i've actually had my best punts with agency escorts. A lot of EE Russians, Czech & Polish - so usually good looking, fit and slender model types. The older ones tend to get surgery and face lifts.
A lot of overdone retouching in picture galleries is off-putting, and yes shop around as many girls are with other agencies, albeit under different names and sometimes different pictures. You will pay a premium compared to Adultwork/indies - but stay away from the escorts who are 300+ bracket, you'll probably find they're not much better/hotter than the £150-£250 girls - they're just higher priced because they're older/more experienced, busier and can demand it. As always your mileage will vary.
...and yes very easy to book! lol
Good luck, and I look forward to seeing a review