I used to regularly see an SP and one day I noticed her rates had gone up, so I asked about the change. She said I could continue paying the old rate, so I did as I saw every 2 weeks for about a year or so. If the frequency of my visits dropped then she might have said you need to pay the new rate as I'm not getting as much income from you now.
It's entirely up to the SP what policy she applies. If the SP is happy that you are a regular client and she is happy to charge you the old rate to retain you as a client, then all good. If she wants to charge you the new rate, then I guess you have to review the situation. If you are happy with the service the SP provides then you'll probably be happy to take a reasonable increase. If the increase is too much and you are on the fence regarding the quality of the service provided, then you may decide to go elsewhere. It's the same problem all businesses have when they increase their charges e.g McDonald's increasing the price of a burger causes them to lose some customers. Imagine if McDonald's said customers who have bought more than 2 burgers in the past 12 months get burgers at the old price and everyone else has to pay the new price. They'd be making less profit on the old customers than on the new ones.
A price increase might cause you to look elsewhere and elsewhere may or may not be as good or better. If you throw your toys out of the pram at the SP because of the price increase, then she is unlikely to want you back. A price increase may result in you reducing the frequency of your visits, which would lead to a reduction in bookings if the SP from you, but they might be happy to accept that as they'll fill the vacant slots with new clients who are happy to pay the new rate.
It makes it easier for the SP to manage their clients and their books as they know all customers are paying the same rates. If they start having to say client A pay £x and client B pays £y then it gets harder to manage and presumably to file her taxes too.
It's entirely up to the SP to set their rates.
If you don't like the rate, then you are free to go elsewhere. Nobody is forcing you to see the SP.
The SP can probably easily replace you, but can you easily replace the SP?