The Derby is fifty shades of downright dangerous. You hopefully already know that. Insurance is very important. Chaps, this really is a one-of-a-kind adventure and you need to pick up the phone and talk to insurers about what you are doing. Many will balk at the word 'race'; you need to explain that this is not a professional event (important distinction from insurer's perspective). Do not put yourself in a position where you could be stuck in Mongolia with a fat medical bill and an even fatter evacuation bill to get you somewhere with the standard of medical facilities you think you deserve.
You need very comprehensive and responsive medical insurance for your treatment/travel for further treatment/repatriation. From an insurer who KNOWS what you are doing.
We are currently reviewing our information on insurance for 2019, so bear with us.