In basically any public place in London, you are likely to be found and have emergency services called and arrive.
Richmond Park is probably the quietest and potentially most remote, but it is also close to several hospitals. There are parts of it where few people go, but you might run into deer.
A hotel room is generally a better bet.
All this being said - yes finding your body would be distressing, but you dying will also be distressing. I'm sure you know that.