Maybe try Bella on ERS in the West End- huge 44FF rack. Big brunette and a BBW in every sense of the word- also very pretty, well groomed and manicured, beautiful lingerie, very enthusiastic and not a clock watcher. Not sure if she just does a BNG or not- currently she's doing an hour FS for 160- worth every penny if you want to spend the time with her. She doesn't advertise a lot and her schedule is somewhat sporadic, but she's a fantastic person and LOVES sex. I haven't seen her in a while, but emailed her recently to try to hook up- our schedules just haven't meshed.
Of course, there's always Jewel (sorry Jewel, not trying to make it sound like you're second fiddle or just routine, by any means- just that you are so well known already with such a great reputation!) in Surrey as well- the girl NEVER disappoints. But not a "hidden gem" as you are looking for.