I think the ballot thing is pretty random. I live in the South East & know diehards that have never got in & people who enter for the craic & shit themselves because they get in. I think the way to go with London if you are determined is to find an obscure charity that's not after £1000000000000.....
Many of my friends, proper runner's and charity chugger love it as a race or 'bucket list event' I've done a few marathons, the biggest being Brighton twice, but never fancied it.