Just to chart how British and Japanese tastes for apple in curry developed:
The first published recipe for curry discovered so far in British recipes is from Hannah Glasse's The Art of Cookery Made Plain and Easy, first published in 1747. Page 101 of 1755 edition, as digitised by Google, has a recipe To Make a Currey the Indian Way, which makes no mention of apples.
From the late 18th century to the early 19th century, as the Georgian era yielded to the Regency in the United Kingdom, the Hindoostane Coffee House in London, opened by Sake Dean Mohamed, became the most famous Indian restaurant, and although these dishes were certainly mild in spice (by modern British standards), no mention of the flavours of apple is made in London's first restaurant guide, The Epicure's Almanack, published in 1815.
Fast forward to 1845's Modern Cookery for Private Families by Eliza Acton, and we see a range of curry recipes, including tripe curry, chicken curry (which includes the mention of grated coconut), country captain and curry balls. No mention of apples in any of these curry recipes, but there is this tidbit about making curries on page 333, under the heading for "Madras Curry Powder":
In India there is always something acid in the mixture, as lemons, sour apple juice, or green tamarinds.
This reminded me of another beloved Anglo-Indian favourite, the chutney. Chutney, under its various spellings, reached the English language in the 19th century, and apple chutney was certainly "a thing". On tracing its history, the 1853 Indian Domestic Economy and Receipt Book by a certain Robert Riddell makes no mention of it (its "love apple chutney" refers to tomato chutney), although it speaks about both English and 'native' Deccan apples. Rather, we turn to Georgiana Hill's How to Cook Potatoes, Apples, Eggs and Fish from 1869, where recipe 192 has apple chutney, where raisins and tamarind are used alongside the "sharp apples". Only a recipes down, recipe 200, is apple curry.
Cut some pared apples into large, but not very thick, slices; mix with them an onion chopped up small ; throw this into a pan of butter, which shake over a brisk fire until it attains a rich brown colour. Take the remains of any cold poultry or meat, previously rubbed over with a mixture of one ounce of curry and two ounces of flour ; stir it about well in the pan, and when the butter is quite absorbed throw in a pint of rich gravy or new milk ; simmer it for a quarter of an hour, and serve with sippets of toast dipped into lemon juice.
It was in the 1860s that the Meiji period got underway, and when the Imperial Japanese Navy adopted curry from the British Royal Navy. JapanToday.com covered the earliest Japanese recipe for curry, dating from 1872, in the 西洋料理通 Seiyō ryōritsū (The Expert on Western Cookery) by Kanagaki Robun 仮名垣魯文. We see the establishment of the roux method, but no apples to be seen.
About twenty years later, the 1891 New York-published Tempting Curry Dishes uses apple and applesauce quite liberally in some of its curry dishes, as well as having a recipe for curried apples. In the East Riding of Yorkshire, one domestic servant working at the Manor House in Willerby has in a handwritten notebook a recipe for curry, involving frying "two sour apples", and it is likely this recipe that has been popularised by the venerable "Mrs Crocombe" on English Heritage's YouTube channel. Interesting, the above BBC article reports a comment from head chef of a Yorkshire-based Indian restaurant chain, Mohammed Aslam:
"Lamb and apples is one of the oldest recipes," he said. "It's a southern [Indian] style of cooking, because what they do in the south is seasonal - what they grow, they use, like apples.
Then again, Mrs Beeton's 1861 tome had no apples in the curry recipes; an 1895 cookbook entitled Anglo-Indian cookery at home : a short treatise for returned exiles makes no mention of apples at all. The flavour profile remained, with recipes for British "colonial-style curry" and the revered coronation chicken. These were gradually superseded by a wave of immigration from the Indian subcontinent and a new wave of British curries through the 1960s and 70s.
Thus, approaching the late 20th century, with apple in curry being a regionalism in India and an anachronism in Britain, it appears that a New England take on curry, through one doctor's obsession with honey and apple cider vinegar, a health fad (バーモント健康法 Bāmonto kenkō-hō, the "Vermont Health System") that took off in Japan, intense marketing in the 1960s by House Foods, and now Japanese-style カレー karē is virtually defined by its use of apple and honey. Until the next change in tastes...!