Since posting this, I tried to take off in the Shoreditch area, where I had done previously (in April 2022), in an area outside of the City of London restricted area and away from railway lines in the area. It's a DJI restricted area now!
I got home, looked at the Geozone map and DJI have taken it upon themselves to add most of London into a made up zone called London NFZ. I overlaid their geozone map over the one from Altitude Angel and marked the expanded areas in Green (left the south west corner out as it's restricted for Battersea Heliport). So now it seems it's necessary to unlock large swathes of London that actually have no legal restrictions on take off or flight.
On top of that, DJI don't appear to restrict take off on the Isle of Dogs (Canary Wharf), which is restricted IRL and I've already had problems with their City of London Airport zone being much wider than the actual one.
Flying in London is already complicated enough, with so many railways and airports and restricted areas, without DJI making it even harder. Do they not want to sell drones to people in the capital?