I was just thinking about this myself because T's wife is raising hers (by £2!) and I am wondering whether he will follow suit (they were previously charging the same as each other).

My view is that if the therapist does want to raise current clients' fees, its' their prerogative and it is fine so long as it is clearly contracted at the outset of therapy. If it's not contracted, it's not okay. And there should be flexibility around current clients' ability to pay.