I was looking to book a room in England last year and was reading the small print. Imagine my surprise when I found out the hotel, Premier Inn, had a policy that they could raise the room rate any time prior to my checking in.

From the premierinn.com website (bolding mine):

Room prices are per room, per night and are inclusive of VAT. Meals are not included in the room price.

You may pay for breakfast at the same time as paying for your room. All other meals must be paid for separately.

Premier Inn reserves the right to review its room prices from time to time. If you make a reservation more than one month in advance, the quoted room price may change before the date of your stay at Premier Inn.

Some hotels may charge a supplement during key events and periods of high demand. For details of our most current room prices please check our website www.premierinn.com or contact your chosen Premier Inn directly.

So depending on the hotel chain, they may be within their rights to charge you more.