...[That particular plotline featuring a day in a cheerful family's life] would do nicely for [live theatre] matinees with lots of women eating tea off trays, but absolutely no use, my dear, for my and Aubrey's sophisticated public.
Dinner theater became popular in the 1950s in the USA, very soon after this British mention, according to https://www.foodservicenews.net/October-2015/Culinary-Curiosities-Whats-the-History-of-Dinner-Theater/ . The historical mention in the LA Times at https://www.latimes.com/archives/la-xpm-2011-jan-07-la-et-dinner-theater-side-20110107-story.html perhaps explains Ms. Thirkell's actress looking down on dinner theatre: "Theater historian Ken Bloom has a scathing view: 'Most dinner theater has been theater at its lowest-common denominator. At the small places, it could be bad ham on stage meets bad ham at the buffet table.'”