No one drinks Mild Ales anymore (not outside the Black Country anyway), but not so long ago London's biggest breweries churned them out in phenominal quantities and Mild was very much of the London drinking scene.
An X ale would be the most basic mild, so an XX is the next step up, hence the 4.0% ABV.
English-Style Pale Mild Ale
ABV:
4.00%
Florence Brewery
London, United Kingdom