One of my favourite hotels, the Corinthia London, had been offering 25% off for the summer which has just ended. They have now added a new offer for 25% if you book by 26 September 2017 for stays until 17 November 2017. The hotel has a fantastic spa as well as some of the most comfortable beds in London. You can read more about the Corinthia here.
If you are likely to eat or drink at the hotel, you may be better with the American Express offer which includes champagne as well as £125 credit towards food and beverage. You can read more about the offer of 25% off here.