Wow, a rare example of government doing the right thing, and getting it. Recently the Ordnance Survey opened up most of their map data. I wrote off for a small selection of maps (they send it to you on super-flexy eco-DVDs) and yesterday they arrived.
What I got was (I think) the whole of England at OS StreetView level, and some other disks of location data (name to map location mappings and so on). The format is really open — TIFF for the maps, and CSV files for most of the other data. Full marks to the OS for releasing this under a CC-compatible license.
I’m still exploring it, but here’s a random snippet of StreetView of Bedford so you can see the level of detail: