Can confirm that Herne Hill Harriers is still going.
http://www.hernehillharriers.org/youngster-star-track/
My children did a camp and enjoyed it - lots of kids up to the teenage years, nice mix. Not the slickest in terms of promotion or organisation, but seems to be a labour of love of Steve's, who runs it. And great value as previously mentioned.
My company Lifemin has what we believe is the most comprehensive list of children's activities/clubs in London, with location and cost details, for when you're looking for less common activities like this, or looking for something close to home. (Herne Hill Harriers is on the list!)
If you sign up for free you get access to the list
https://www.lifeminco.com/2020/05/11/su ... ctivities/