Containers

At a time when resources of every kind were short, it made sense on many levels to favour cask beer over bottled. 

With both glass and rubber for stoppers in short supply, there were some obvious disadvantages of bottled beer. (In 1943, 80% of the world's rubber supply was under Axis control.)  It also required more energy and manpower to produce and was heavier and bulkier to transport.

But cask beer wasn't without its own problems. Basically, the casks themselves. Though a wooden cask has a long life, it does require regular maintenance. And replacing staves requires a supply of oak. For a long while UK brewers had preferred Memel oak from the Eastern Baltic. Even before Germany invaded the Soviet Union supplies had been cut off.

"Containers
The search for every means by which economy of transport could be furthered naturally led at quite an early stage— two years ago and more—to consideration whether the replacement of bottled beer trade by cask beer was not feasible. Cask beer makes substantially less demand on transport. The immediate answer was the lack of sufficient casks in the country to enable any material expansion in the use of this form of container. Since that time there has been no improvement, but perhaps on the whole there has not been any further serious deterioration. Before the war there were considerable stocks of cask staves — almost entirely of Memel oak — in the country, and these delayed the full effect of the cessation of supplies until the first year or two of the war had passed. The steady drain on cooperage labour through calling-up was already having a serious effect by the time stocks of Memel staves became exhausted. In some few cooperages it had long been customary to use English oak for casks, but efforts to extend the use of this material did not meet with very much, success. It called for considerable skill in working, owing to the hardness and nature of the grain, and skilled labour was short. The Government, realising that the cask shortage would rapidly grow worse and would threaten the maintenance of beer output, made available a steady supply of American oak staves. This material had the serious drawback that the casks must be lined with pitch in order to avoid the beer being affected, but for well over a year now American staves have saved the situation. Supplies have been enough to keep cask stocks substantially up to the level of early last year. Lack of cooperage labour would in any event preclude much improvement over that position. Large stacks of casks still remain which cannot be brought into service again without repair, while the demands of the Services and the public generally and the growing delay which occurs in getting empties back again, particularly when, as they often must for transport economy reasons, casks of beer are sent by rail, are making it more and more difficult to make the available casks go round."
The Brewing Trade Review, August 1943, page 243.

In normal times British brewers would never have used American oak as it imparted too much flavour to the beer it contained. Unlike today, brewers wanted to avoid any trace of oak in their beer. Its presence was seen as a fault. But, with the supply of Memel oak dried up, brewers had little choice.

When supplies of American oak in turn began to evaporate, brewers had to turn to a more local source.

"That is the general position at present. There are, of course, many exceptions. Some brewers are fortunate in the possession of their own cooperage department and sufficient skilled men over military age to handle their work. Others are in much worse case. The fact that the cask position is as good as it is at present is in no small measure due to the co-operation of the cooperage trade, individually and collectively through their Association.

Now the brewing trade is threatened with a serious reduction in the supply of staves in the future. There is a shortage of American oak, and present indications are that early next year American stave supplies will be severely curtailed. It is believed that the seriousness of this is appreciated by the Government Departments concerned, and that efforts are being made to relieve the position. It is hardly likely, however, that supplies can be maintained at the level which has prevailed in the last 12 months, and it will be necessary for an increasing proportion of cask requirements to be carried out in English oak. Here again there is no bountiful supply, but such quantity as can be provided will have to be used in substitution for the American oak. One unfortunate result will be that greater difficulty of working probably means longer time in making up a cask, and consequently still further attenuation of the already severe shortage of skilled labour."
The Brewing Trade Review, August 1943, pages 243.

This is first time I can recall hearing of English oak being used for casks, though I'm sure that was what was used if you go back to the beginning of the 19th century or earlier. Why was it harder to work tha American or Memel oak? Was it harder? 

There was one simple solution to the shortage of casks - rotate them through pubs quicker.

"The position calls for the careful attention of all breweries if serious consequences to the output of beer in the near future are to be avoided. The first consideration which comes to mind is the achievement of the quickest possible turn round of casks. No doubt steps have already been taken by most breweries to get their casks returned more quickly. The Ministries concerned are alive to the urgent necessity of such quick return and we believe they are working to this end so far as railway traffic is concerned—not only with beer casks but with returnable containers of all kinds, for the same difficulties are present in other industries. It remains for the brewery to deal with other sources of delay and to do what is possible to ensure that casks are sent back from licensed premises and other customers as soon as possible after they are empty."
The Brewing Trade Review, August 1943, page 244. 

Zoning - only delivering to pubs close to the brewery would have helped casks get back to the brewery quicker. But other wartime fuel economy measures didn't help. Brewers reduced the number of deliveries and generally only dropped off beer - and collected empties - once a week.