I ran past him at 21 miles and I finished in 4hrs32min. He should be able to walk the course and collect money for charity but not completing the full course should mean no medal.
I always stay at the Ramada Docklands. Its next to the Excel Centre for registration, it is a 2 min walk from the Docklands light railway stop which gives easy access to the start. The only downside is getting back from the finish, but the quality of the hotel makes up for that!!!