The diamondgeezer site is one my favourites along with ianvisits.co.uk and londonreconnections.com - there's all sorts of content on there about the absolutely most 'niche' things you can think of in London.
And then you find out about things like the Pocket Parks, what's on in London, mudlarking, and so on. Odd things like the Cannon St. to Mansion House tube run, and then a parkour version to Barbican https://www.youtube.com/watch?v=tXMPRK2LQAE
Too many to list.