Here's a link that tells more about it: Red Nose Day May 21st 2015
There is some kind of concert or show they are doing that will really be the main fundraiser. I guess the whole "red nose" thing was just to draw attention to it and get people interested.
I'm really never sure about these things--the ice bucket challenge, red noses, wear red or blue or green or yellow for <choose your charity or cause here> sort of thing--but I guess it DOES help raise money and awareness of the cause, or they wouldn't keep coming up with this stuff.
At any rate--even though I *loathe* clowns with every fiber of my being, I decided to participate.
Comic Relief UK started the benefit. I suppose a red nose relates easily to comedy, which is the chosen tool for the campaign. Personally, I think of Robin Williams in the movie "Patch Adams."