Geograph - photograph every grid square

The A4 (Great Western Road) runs 127 miles from London (Holborn Circus) to Avonmouth. It heads west out of London through Slough, Reading, Newbury, Marlborough, Bath and Bristol. Today it is paralleled by the M4.
Wikipedia: Link

A few years ago I read a book by Mark Mason "Walking the lines" in which he walked the route of the London Underground lines, overground. I will repeat this visiting all 272 stations (some more than once). My route tries to follow the line of the railway as closely as possible, using footpaths and roads, deviations are made to make the walk more "interesting".