Some parlours do it one way, some do it the other way. The parlour receptionist will explain as they take your cash.
Assuming a 30 minute punt and a £50 base price, they'll either ask for £20 off you and expect you to give the remaining £30 to the WG or they'll ask for £50 off you.
In either scenario the WG will usually expect you to agree extras with her in the room, such as OWO for £10 and Anal for £30.
So some places you'll hand over £20 to reception then £70 total consisting of £30 + £10 (owo) + £30 (anal) to the WG. £90 in the end.
Other places you'll hand over £50 to reception then £40 total consisting of £10 (owo) + £30 (anal) to the WG. £90 in the end.
Sometimes the WG will want you to pay extras at the start of the booking, but sometimes they'll collect the extras at the end (or the whole of their fee). Most of the worst OWO and Kissing I've experienced have been from WGs who collect the money for it up front.
If you don't discuss specific extras and the price with the WG before doing them you've got a possible outcome of finding out they expect more for the extras than you were planning to spend. But maybe some WGs will find you so incredibly charismatic and handsome they won't charge you as much as you expected for the extras.