I came across the notion of British Israelism entirely by chance, and found the very notion rather surprising. In fairness, it's not that different from other pseudo-historical nationalistic repurposing of concepts that occurred through history such as the Nephite Ancient Israeli settling in the future USA, according to the Book of Mormon,, Hyperborea, St. Iago of Compostela, the Aryan Race, the Aeneid, and so on.
However, I remember having been utterly confused at the idea of a song named after Jerusalem and, at first glance, ostensibly about the "Holy Land", being the unofficial British National Anthem. I thus now left wondering whether there's a causal link between the two, and how this idea may have directly or indirectly influenced the UK, US, and Commonwealth to this day. I'd appreciate any feedback, since I intuitively get the impression that this idea explains many peculiarities that have surprised me about Anglo-saxon thought over the years, but my admittedly atrophied intellectual prudence strongly suggests that I don't jump to conclusions when finding a Shiny New Thing.
Because identifying those relationships are very interpretive, I'll mostly focus on the context surrounding its widespread adoption, which occurred during the First World War. To quote Jason Whittaker,
The poem’s true afterlife, however, begins in 1915, when the Poet Laureate Robert Bridges included it in the anthology The Spirit of Man and then commissioned Parry to compose music for the text. It is from this point onwards that ‘Jerusalem’ can be seen as part of English national life and as a nexus of agencies seeking to use the text to perform a series of often-contradictory ambitions.
This Poem's timing in a well-known author's compilation, combined with the cultural context of WW1, is crucial. The poem invokes imagery of a holy land to promote the greatness of a largely-Protestant England, which further aligned with the ongoing nationalist wartime rhetoric. Indeed, Perry was originally commissioned specifically to set this "And did those feet in ancient times" to music for a patriotic rally in 1916. The call to arms in the third and fourth stanzas fit nicely in a time when many English people felt that their home was facing an existential threat. It's worth noting, too, that both the Labour and Conservative parties invoked the hymn to appeal to a great, forgotten era post-WW2. In some cases, to celebrate nationalist causes; other times to celebrate the beauty of pre-industrial England. In either case, this poem/hymn resonated with a lot of people.
It is definitely a piece that lends itself to nationalist rhetoric, and the original poem does invoke religious imagery. However, I personally lean towards the "right time, right place" idea of it reflecting ongoing wartime struggles, people resonating and appropriating it, and then subsequent politicians invoking this collective idea. I think somebody could argue that there is a correlation between British Israelism and the hymn's popularity - indeed,
Further Reading
Mark Chapman. “William Blake, Hubert Parry, and the Singing of "Jerusalem".” The Hymn 62, no. 2 (2011): 41.
Whittaker, Jason. “Blake and the New Jerusalem: Art and English Nationalism into the Twenty-First Century.” Visual Culture in Britain 19, no. 3 (2018): 380–92.