As others have said, there is no set etiquette - very much up to the girl. In my experience, if you agree it at the very start of the session, you will almost invariably get the rate for the time you are asking for. But the longer you leave it, the more likely the SP is to treat it as a new booking, depending on how generous or not she feels (and how good she is at mental arithmetic!).
If you are only prepared to pay the extra £80, you are better off stating that up front ("If I pay you the extra £80, can I stay for a full hour?). Rather than something like "Can I stay another half-hour" (= another £120 in her head) and then attempt to argue over it.
If you have to haggle over extending the time, and then she has to check with her pimp/agency that it is okay to extend, then you have to find the money, she counts it and then goes away to stash it, can all be a bit of a mood killer. Many SPs also have routines as to what they do in given time bookings, and can get thrown if you ask to extend (do you want her to do exactly the same all over again?). As previous poster said, if you have had a good experience, you are usually better going back a second time for longer.