You don’t need to fly drones over nuclear sites to find out all about them… just have lunch at the local pubs for a few months, you’re all set.
(Should explain, really: the book includes a section about AWE Aldermaston that’s so accurate it triggered an investigation into who had been leaking secrets. Turned out Seymour spent months having lunches at the local pubs just listening to the workers moaning to each other about their jobs.)
Such specific reports, the swarm was some time between 2014 and 2020
When you read the actual Freedom of Information request mentioned, which I think has been reported prior on this forum, it is much less dramatic.