All proceeds from the sale of the burger will be donated to Help a London Child charity, which helps children experiencing abuse and poverty.
To me, this seems like a charitable fund raiser. At most of those, you pay over the top for the item, it is almost like a token souvenir, when really you are donating to the cause.

Of course, they are trying to get in the Guinness Book, so that shades it a bit; but I bet quite a few of those 50 who purchased where considering the charity involved!