What was being drunk in Somerset in 1888?

At least in the pubs served by Hancocks of Wiveliscombe. Maybe they were atypical. But the short answer is: Mild Ale.

Three-quarters of their output was Mild. Way more than any other beer. Yet they only brewed the one. At a time when three or even four wasn't uncommon. While the two Pale ales could only manage 7% between them. Implying a the area around the brewery wasn't wealthy. Assuming that's where they sold their beer.

Being honest, I've no idea what GA was. The more I look at it, the more it looks like a weaker Mild Ale. Not having found a price list yet, I don't know what it was marketed as. And what does GA stand for? Golden Ale? Guinea Ale? Any suggestions?

I've found plenty of beers described as Guinea Ale in price lists. Almost always a Pale Ale costing 1s 2d per gallon. While, based on the OG, this GA would be 10d per gallon, at most.

The Porter was a surprise. Even if they only brewed it a couple of times.

Long answer: bucket-loads of Mild Ale, a few teaspoons of Pale Ale and Stout and a pint pot of something that I don't know what the hell it is.

Almost forgot. Hancock GA is full of No. 3 invert sugar. What more shouts Mild than that?

Hancock output second half of 1888
Beer	Style	OG	Barrels	%
BA	Pale Ale	1060.1	313	2.53%
BB	Pale Ale	1052.6	559	4.52%
GA	Pale Ale	1039.3	1,406	11.38%
Porter	Porter	1052.6	77	0.62%
Stout	Stout	1066.3	689	5.58%
XX	Mild Ale	1050.2	9,313	75.37%
Total			12,357
Source:
Hancock brewing record held at South West Heritage Trust Somerset Archive, document number DD/HCK/5/2/1.

The figures come from the brewing records covering the period 1st June to 29th of December.. Pretty much exactly six months.