David Kane, @kanedr on Twitter has worked on this extensively and will be able to provide a lot of insight. For many years, he *was* the NCVO Almanac!

Off the top of my head is to bear in mind that these are registered head offices and many charities have branches outside London. I don’t believe the Charity Commission has this data – unclear why not! So OpenCharities may also be missing this.

Also: you’re missing a huge swathe of charities in Scotland, Northern Ireland, and unregistered groups. Charity regulation is a devolved matter and the criteria for registration leaves out smaller, local charities who might be registered with HMRC for gift aid, currently a closed list.