The government issued brewers permits to buy a specific amount of barley or malt. This was based on a brewery's malt usage in the last 12 months of peace.
"11. Regulation of Barley and Malt Purchases.— A joint committee with the Maltsters’ Association was formed in April, at the instance of the Ministry of Food, to frame a scheme for the regulation of purchases of the 1940 crop of barley for malting. The resulting scheme was put into operation under the Barley (Control and Maximum Prices) Order 194O (S. R. & O., 1737), with the object of regulating by means of permits issued by the Ministry of Food the quantities of barley or malt to be purchased by each brewer according to his estimated requirements for brewing up to the time when the next crop should become available as malt. The scheme involved no restriction of normal methods of buying barley or malt beyond the limitation of each brewer s total purchases to the quantity for which he held the permit of the Ministry of Food.
12. This method of regulating the purchase by brewers and maltsters of malt and barley has remained in operation throughout the war, although the reason for its necessity has gradually changed from one mainly of conserving barley supplies to one of ensuring that the limited malting capacity should bo fairly shared throughout the country. As the acute shortage of malting labour is relieved, the necessity to continue this control will diminish, but it is unlikely that anything like a sufficient labour force will become available in time to be effective until the malting season restarts in the early autumn of 1946. In the meantime, a brewer is permitted to buy enough malt (or barley to make the malt) to bust until 30th November each season not a very wide margin of carry-over when new season's malt can scarcely be ready until the latter part of September. In arriving at the quantity, the use of sugar, flaked barley, etc., is taken into account. In the case of Scottish breweries, an extra month’s supply up to 31st December is allowed to compensate for the later date at which Scotch barley becomes fit for malting."
Brewers' Almanack 1946, pages 128 - 129.
The rules meant that the government had a considerable degree of control over brewers' grists. Also rationing brewers to 70% of their pre-war sugar usage and insisting on a certain percentage of unmalted grains, usually flaked barley. I'm sure some brewers weren't very happy about having their hands tied in this way.
On the other hand, the rules did ensure everyone got their fair share of brewing materials. I'm sure German brewers would have been delighted to have such luxury.