Following a tweet by dominijk, I used Ordnance Survey’s Open Roads data set to create a polygon of the M25 as this is commonly used as a feature boundary in the media as an unofficial region.
I then applied the same technique to the Manchester orbital motorway, the M60 as this is similarly used.
You can download both polygons, in ESRI shapefile format, on the links below:
As open data derived work, this data may be used subject to the terms of the Open Government Licence (OGL) Version 3. Please attribute the source with “Contains OS data © Crown copyright and database right (2019)”