Excise duty

After 1880, the way of calculating exvise duty on beer wad an odd one. It was charged per "standard barrel" rather than by the actual volume of liquid.

This text explains it:

"From 1880 to 1933 the unit of charge was the "standard barrel," viz., 36 gallons at the standard specific gravity, which was 1057 deg. until 1889 and 1055 deg. thereafter. In 1923 the duty was reduced by a rebate of 20s. (or less on beer of very low gravity) per "bulk barrel," viz., 36 gallons whatever the gravity. This method was adopted in order to facilitate a uniform reduction in the retail prices of practically all beer, and the rebate was continued concurrently with subsequent increases in the charge."
Brewers' Journal 1940, page 49 (published January 17th, 1940).

But in the early 1930s, there was a subtle change to the system.

"In 1933 the duty was reduced and remodelled. Before the Budget of that year the Excise duty was £6 14s. per standard barrel minus the rebate referred to. This was altered to £1 4s. per barrel of a specific gravity up to and including 1027 deg., plus a further 2s. for every degree of gravity over 1027 deg. Thus the unit of charge was shifted from the standard barrel to the bulk barrel, and the charge was made to vary with the gravity of the latter. The reduction was calculated to be sufficient, in most cases, to permit of a fall in retail prices of 1d. per pint and to leave a margin to cover a rise in gravity."
Brewers' Journal 1940, page 49 (published January 17th, 1940).

This new system was a huge disincentive to brewing anything under 1027º. Because whether your beer had an OG of 1010º or 1027º, you still paid £1 4s. per barrel. Sure enough, I don't think I've come across a single beer with an OG below 1027º after 1933. While in WW I, there were beers as weak as 1010º.

"The Customs duty on imported beer exceeds the Excise duty by 1s. 3d. per bulk barrel, of which 5d. is to countervail the cost, to the home brewer of the licence duty and the Excise restrictions to which he is subject, and l0d. to countervail the Customs duty on imported hops. On imports of non-Empire beer, which consists largely of lager beer, there is also a surtax of £1 per bulk barrel imposed in 1936 as a measure of protection to the British lager beer industry."
Brewers' Journal 1940, page 49 (published January 17th, 1940).

I'd seen an extra tax on imported beer suggested for the purpose of helping Lager brewing in the UK. But I hadn't realised that it had been implemented. As mentioned in the article, UK beer imports, other than those from Ireland, were almost exclusively in the form of Lager.

And finally, a little mention for Black Beer, that weird and obscure stuff:

"On black beer, which is a special high gravity product, exceeding 1200 deg., in the nature of a syrup, the duties and rebates have remained unaltered since 1923. There have been no imports for many years, and the home production is small, having been 378 bulk barrels in 1938-39, equivalent to 1,668 standard barrels."
Brewers' Journal 1940, page 49 (published January 17th, 1940).

With only 378 bulk barrels produced, it was just about extinct.