On the Sustrans (Scotland) site you can now obtain a complete list of all milepost locations for the entire UK. The raw data is supplied as a KML file, which you can import into GIS and mapping systems.
I had a go myself, and managed (to my surprise) to create a map using the KML locations, the Open Street Map data, and the umap website:
If you make anything interesting out of the Sustrans data, add a link to it in the comments!
Sustrans Scotland are leading the way in providing open data for the National Network. Although the milepost page and data for the UK is available only on the Scottish web page at the moment, it will soon be available on the web pages for the other regions.