English oak was in short supply and since the 19th century most of the wood used in making casks had been imported:
"While the question of new casks and a supply of timber for repairs may not be a very difficult problem at the moment it is likely to assume large proportions if the trade remains normal and is not subjected to much restriction. The timber found to be most suitable for brewing industry comes from Russia and Poland, being shipped from the Baltic ports, but this source was cut off as soon as war broke out and brewers will be obliged to look elsewhere for their supplies. English oak has proved to be eminently suitable and is considered to be even superior to Russian oak, but unfortunately the available supplies are far too small to satisfy the demand.
During the last war supplies came from the United States and Canada, but although this oak proved quite suitable for cask making trouble was experienced when it was put into use. The principal objection was the rather unpleasant flavour it imparted to the beer, which the usual methods employed in the cooperage for treating now casks before they are put into use, failed to eliminate."
Journal of the Institute of Brewing Volume 46, Issue 4, August 1940, page 277.
This is another piece of evidence that suggests British casks were still unlined, unlike casks on the Continent, as beer came into direct contact with the wood. From evidence such as this, I've come to the conclusion that British casks were never regularly lined.
English oak was also too expensive:
"He [Mr. L. C. Thompson] had had a few excellent casks made recently of English oak, but the cost was about double the present cost of Memel."
Journal of the Institute of Brewing Volume 46, Issue 4, August 1940, page 279.
What was it that caused American oak to taint beer? I'll leave that to Prof Groom:
"Prof. Groom found that the tainting of beer was not due to fungi or bacteria acting in the wood or gaining access to the beer through the wood, but was caused by the entry into the beer of some natural constituent extracted from the wood. The conclusions drawn from the examination of a number of samples was that wood may be unsatisfactory because it is an inappropriate species, was felled at the wrong season, or that it consisted of heart wood that was too young.So it was the wood itself, rather than anything nasty lurking in it. If beer would pick up a taint from the best American oak in as little as ten days, how on earth could the cask have had any internal lining? The life of British brewers would have been much simpler had they lined their casks. Then they could have used any old oak, as long as it was strong enough.
. . .
The best quality of American white oak was found to give the most satisfactory results, and while very little difficulty appears to have been experienced where the casks were used for quick trade, the flavour invariably made its appearance if the beer remained in the casks for more than 10 days. These researches must have drawn attention to the variability of American oak for cask-making, and it is probable that future supplies may show an improvement in consequence." Journal of the Institute of Brewing Volume 46, Issue 4, August 1940, pages 277 - 278.