IMO £100/hh is steep so an increase of £20 to £120 is OTT however we need to bear in mind that there will be regional variations in price so what is OTT in one area may actually be the norm in another and at the end of the day as punters we can choose to stay away.
The other thing here is that without knowing who the girl is and what services are included then it's hard to judge whether or not the price is high.
For instance if the girl is all inclusive and offering anal; watersports; CIM etc etc and an 18 year old stunner then perhaps £120 is actually a bargain
Probably best for the OP to take the £120 and if it was me i'd mention that i'd seen her price had gone up after i'd booked along the line of "Oh am I right in thinking you have put your price up? Just noticed when I looked at your AW profile after I booked"
That way you broach the issue up front in a non confrontational way so you're not complaining or pushing back against the increase but it does prompt a response from her to either accept the money; decline the money and keep you on the previous rate or decline it for this punt but future punts will be more.
Once you have your answer then you have a decision to make whether to keep her as a regular or not or just to punt her occasionally and if she comments that you don't visit her often any more then you can let her know it was the price increase that made you reduce the frequency of visits, she then has a choice to make