No Reduction In Beer Output

The restrictions imposed on brewers in WW II were far less Draconian than in WW I, but kicked in far earlier.

That was mostly dictated by the different progress of the two wars. In the first conflict, UK food supplies only came under serious threat almost three years in, in spring 1917. In WW II the serious problems started after less than a year, after the fall of France in the summer of 1940.

Which is why in early 1941 brewers voluntary reduced their grain usage:

"No Reduction In Beer Output
BUT GRAIN CUT MAY MEAN LIGHTER BREWS
The ten pear cent. cut in grain for brewing implies no restriction on the quantity of beer produced, states the "Brewing Trade Review", official of the Brewers' Society.

"It is the desire of authorities the that British beer shall to be available where needed," the journal states,

"Under present conditions of effort and strain it would be indefensible if the British worker were prevented from obtaining that refreshment and relaxation which he is accustomed and for which the present circumstances accentuate the need."

With modern vastly improved methods of brewing, the journal says, it is probable that the saving in material will be much more real than apparent that the beer drinker eill have little conscience of the change. Some beer of heavier gravity may, however, disappear from the bar, it is added, and be replaced something lighter."
Morpeth Herald - Friday 07 March 1941, page 6.

You can see the effect on Barclay Perkins standard Bitter, XLK:

Barclay Perkins draught XLK 1939 - 1945
Date	Year	OG	FG	ABV	App. Atten-uation	lbs hops/ qtr	hops lb/brl	dry hops (oz / barrel)
22nd Jun	1939	1045.8	1014.5	4.14	68.35%	7.50	1.35	3.00
11th Jun	1940	1042.8	1012.0	4.07	71.96%	7.50	1.27	3.00
22nd Apr	1941	1042.8	1011.0	4.20	74.28%	6.00	1.02	2.00
8th May	1942	1035.3	1009.5	3.41	73.09%	5.61	0.82	2.00
2nd Dec	1944	1035.7	1010.5	3.33	70.59%	5.29	0.77	4.00
7th Apr	1945	1035.4	1012.8	3.00	63.98%	5.09	0.75	4.00
25th Jan	1946	1035.3	1010.0	3.35	71.67%	7.00	1.04	4.00
Sources:
Barclay Perkins brewing records held at the London Metropolitan Archives, document numbers ACC/2305/01/623, ACC/2305/01/624 and ACC/2305/01/626.

You can observe the typical WW II gravity decline: a small fall in 1940, a larger fall in 1941 and then stability until war's end.

Something similar happened with Whitbread's Bitter:

Whitbread PA 1939 - 1945
Date	Year	OG	FG	ABV	App. Atten-uation	lbs hops/ qtr	hops lb/brl
27th Sep	1939	1048.2	1012	4.79	75.10%	7.33	1.41
30th Apr	1940	1047.2	1011	4.79	76.69%	7.33	1.44
14th Aug	1940	1043.1	1011.5	4.18	73.32%	7.25	1.34
6th Feb	1941	1042.9	1015	3.69	65.03%	6.54	1.11
30th Jan	1942	1040.0	1011	3.84	72.50%	5.85	0.97
29th May	1942	1038.6	1009	3.92	76.68%	6.03	0.96
3rd Apr	1943	1038.5	1009.5	3.84	75.32%	6.03	1.01
22nd Feb	1944	1039.1	1010.5	3.78	73.15%	6.13	1.00
4th Apr	1945	1039.4	1010	3.89	74.62%	6.03	1.01
Sources:
Whitbread brewing records held at the London Metropolitan Archives, document numbers LMA/4453/D/01/107, LMA/4453/D/01/108, LMA/4453/D/01/109, LMA/4453/D/01/110, LMA/4453/D/01/111 and LMA/4453/D/01/112.

Both beers lost around 20% of their gravity over the course of the war.