New Large Scale Ordnance Atlas of the British Isles
The most prominent feature are the bright coloured for the Parliamentary Divisions, as used 1868 to 1885. There is no page/sheet number in the top right corner, so maybe it is not from an atlas at all - but has no signs of having been folded either.
More details on Bacon's maps are on the Publisher's page.
Scale correct at 10 miles = 62.7mm, or 4.0 miles/inch, ie 1:255,000.
The railway from Southend to Shoeburyness is the latest shown as a solid line (opened 1884), but there are then many later lines shown by parallel lines which presumably means "planned", eg Barking to Upminster (opened 1885), Upminster to Laindon and on to Pitsea (opened 1886 & 1888), and Grays Thurrock to Upminster (opened 1892) and Upminster to Romford (opened 1893).