The answer to your question is what agreement do you have with them? If it says they can do it they can. If it says they can cancel or amend bookings they can. If there is no agreement then there could be an implied contract that all bookings are same rate. Contact AOP or your legal advice organisation to see where you stand.
And it has just been announced that NHS sight test fee increased to £21.31 backdated to April.
Does everyone know the claim for for CET now available on Internet at