I was at Emmaus today and asked about price policy and bartering negotiating. The 'rules' are as follows:
* At least two people must decide on the price of the goods. Unusual or valuable items need more input, often advice from outside the establishment. Price is not any more than 30% of the item new or full retail value and often offered as a bargain if in condition 'as seen'.
* Prices are not agreed or 'offers' accepted face to face with the client. If goods stay any length of time without finding interest the price is reduced for a while. Beyond that they are recycled, where possible.
As for the OP question a charity shop may have the set up of a charity business or local/specific interest. I don't know the need for training and scrutiny of staff/volunteers in the UK. The shop itself has to support the rates/rent along with all other costs when occupying premises. Shop prices have to shoulder at least that. I don't see any surprise that they have to raise prices to stay open.
My rare visits to old haunts in England tells me that a charity shop, or several, helps prop up high streets and may not be really what communities should be about but that starts another line of political discussion.........