BJCP 11B
A luscious, malt-oriented brown ale, with a caramel, dark fruit complexity of malt flavor. May seem somewhat like a smaller version of a sweet stout or a sweet version of a dark mild.
Increasingly rare; Mann's has over 90% market share in Britain. Some consider it a bottled version of dark mild, but this style is sweeter than virtually all modern examples of mild.
Mann's Brown Ale (bottled, but not available in the US), Harvey's Nut Brown Ale, Woodeforde's Norfolk Nog
You can view the complete style details on the BJCP website. The 2008 style guide is obsolete but there is a similar 2015 style that you can view below...
OG: 1.033-1.042
FG: 1.011-1.014
IBU: 12-20
SRM: 19.0-35.0
ABV: 2.8-4.1%