I suggest you just find a few girls who take your fancy and email them with an offer.
I have found £50 to £80 to be a reasonable price range for owo. (bearing in mind I like them to come to me)
Best bet in my experience is to see the girl for a full hour first to get a rapor you can then ask her if she's do 30 min bookings with you just for oral.
Or ask up front if they can do a 30 min booking for OWO.
The majority will agree, unless they are pimped.