I've started to acquire and tag administrative borders. In addition to the complete polygon for Rutland in the DB, which was already there, there is now a complete polygon for Leicestershire (traced from NPE), and also one for the London Borough of Hackney. Tower Hamlets isn't far off, either.

For the London Borough of Hackney, I actually went out onto the streets, starting at Finsbury Park, where I know it touches Islington and Haringey, and walked the entire border, without reference to existing maps, merely by looking at street signs and dustbins and parking meters. It should be pretty accurate, except in a few places where the situation on the ground was confusing. Maybe I could do a follow-up expedition, knocking on people's doors and asking them who they pay their council tax to.

Most of the Inner London borough boundaries should be possible to get this way. In less urban areas, we'll have to rely a lot more on inferences though.

I've not tagged these as areas, but as stretches of boundarie, with the tag boundary=administrative, and then left:district, right:district, left:county, right:county as appropriate. You can fetch out the entire polygon for Hackney by looking for all the segments with left:district=Hackney or right:district=Hackney.