Is £600 for night at Mandarin Oriental Hyde Park Hotel a good value of money?

That depends on your budget and priorities. If you like luxury stuff and you put as a target getting to know british culture in quintessential way a price will be worth it for you. If you are average tourist who don't pay attention for all of the material things you won't think that a price is a good value of money. To sum up before you decide if the price is worth staying in this hotel you need to focus on what is important to you if you want to stay here.