Sustrans are great, but their online mapping isn't, and this FAQ response doesn't make sense to me....
Why don't Sustrans use Google Maps or OpenStreetMap?
A Google or OpenStreetMap API (a map that can sit within another web page) re-loads the data attached to it each time you move or zoom the map. Our dataset is over 80Mb so this would run very slowly.
We use Ordnance Survey 1:10 000 and 1:25 000 scale maps as our backgrounds for several reasons:
They are based on the same map product and so will match up with each other at each zoom level. This means that routes will appear correctly against the different backgrounds. For example, a route plotted down one side of a carriageway against the 1:25 000 will appear on the same side of the carriageway when viewed against the 1:10 000 background.
Google mapping is not consistently detailed enough in all areas of the country for our needs. Also areas of shadow created, for example by tree-lined lanes, can obscure important information for cyclists and walkers. We believe that Ordnance Survey backgrounds give much better detail in both rural and urban areas.