OP Plenty of bog standard comprehensives study An Inspector Calls as well as your nephew's grammar school FWIW. It's a great play and I love it but it is by a dead white man (I'll forgive him because he is a leftie) like so much studied in Eng Lit, sadly. I absolutely reject the idea that grammar and private schools will choose more literary texts - nor should they if that means only Dickens and Priestley.
Your DD will also study Shakespeare and a 19th century text like some literary Dickens alongside a poetry anthology and will have to talk about unseen poems as well. So all is not lost. Some of the poems will be modern but many will be old as well.
I don't think that older texts are the only ones that are literary and I am sure you don't either, but it does come across a bit that way.
DD took Eng lit to degree level and only then did she study anything by someone living and non white (tho Anita and Me is also on the GCSE list). It;s a shame and can't be helpful in encouraging non-white female students when they don't see themselves in the books they are reading and studying.
Yes, Sign of Four is on the 19th century list. I imagine the idea is to appeal to boys who may not enjoy Jane Austen or Charlotte Bronte. In fact most schools choose Christmas Carol as the shortest and most accessible text. DD at least did Frankenstein (by a woman! yay!).
Can you tell I feel strongly about this? I very much agree with PPs saying that Eng lit should be tiered (as should Eng lang) and then perhaps we could have two different lists of textx, with maybe excerpts of Shakespeare at foundation, and some more accessible novels from the last 100 years.