Anthony (who's doing good work in his PhD project) also doubles as a runner and has written a nice post for the Significance website.
Clearly (just look at the numbers!), for many people this is a serious issue $-$ the fact that you can't run officially a big Marathon such as London's unless you've been lucky enough to win your place through a ballot.
I have to say my only experience with long-distance running was a few years back at Florence's Marathon, for which I was not officially registered $-$ for that matter, I wasn't even planning on finishing it, just do a bit of the whole thing and then go back home, so I guess it didn't really matter that I didn't get a medal or something...
I'm not sure that guaranteeing a place to somebody who's been turned down a sufficient number of times in a row would solve the problem, though $-$ people must get fed up with the wait?