If you are coming for a short time and have that kind of budget - I would suggest trying an agency. Booking process is much simpler with agency (as if you call girl direct, she may not be free to take call so you have to try again)
Few agency girls are well reviewed here and personally I tend to find the agency process more smoother - though ultimately it does depend on the lady always
If you have £300/hr to blow you should be looking at the elite agencies to start with, crosschecking back here for UKP reviews. Then do a search on AW using £200-£400 as a filter and see what turns up. Sometimes you can get a good agency girl working indie at a lower rate.
This is bad advice. Just because someone HAS £300 to spend, it does not mean that it is a good idea to waste it. There is little or no correlation between price and quality of fuck. By hiring a £250 escort, you are giving £100 to a pimp called Sergei or Kevin and getting a random £150 escort called Svetlana or Jade. Just book a well reviewed independent escort.
There is no such think as an Elite agency. A possible exception is if you want to fuck a famous pornstar. Otherwise, the girls at the Elite agency have 2 legs and a pussy just like all the other escorts. And they might or might not give good service.
(Ok to be fair, the part of the advice to use UKP ratings is right - so basically the girl being on the books of a pimp is neither here or there)