Ordnance Survey
have released some data for wider freer use.

Streetview (raster data)
which could already see on Where is the Path? website which used OS Open Space API but would often run out of allowed hits so won’t now I guess
And with a new license can now be used more widely, and mashed up with a wider amount of data in wider settings and new datasets derived from it.

Contains Ordnance Survey data © Crown copyright and database right 2010

Merdian2 (vector data):
Merdian is not particularly complete lots missing or truncated it seems with junctions abbreviated and other bits roughly done, but maybe that is just that representation of data on that site. But it does cover whole country.

http://oliverobrien.co.uk/2010/04/accuracy-vs-completeness-osm-vs-meridian/

Compare Maps

Coming end of May OS District (Vector)
This looks like will be most useful to Open Street Map

Gazetteer to look up placenames

Also include is Royal Mail Postcode locations in the Code Point data. See
CodePoint Postcode areas on the map.
Will the whole Postcode Address File which has each address be next?
Which Ordnance Survey use as a basis for their Address Layer 2.

Open Street Mappers discussion

Mirror of data

Also, see Gov’t Data site

And there are always those old OS maps over 50 years old that can be freely copied and data derived from to be freely used.