I'm not (quite) the oldest person on this forum, and it doesn't make my opinions any more correct, but I do have some personal memories of the National Anthem over the last few decades.
Back in the 1960s and well into the 70s the music of the National Anthem was played at the end of the final movie of the day at each cinema screening. No-one was expected to sing the words, but the general expectation had been that cinemagoers would rise from their seats and stand still. Greadually during this period, and probably more in cities than in rural areas, people either remained seated or rushed for the exit, possibly provoking outrage from the traditionalists, so the Anthem was phased out in most (all?) places.
I would suggest not singing the N.A. is a bit like, at a church funeral, some of the congregation either not singing the hymns, or just pretending to sing, and ditto the Lord's Prayer: one is not there to draw attention to one's self, so you go with the flow for the sake of those for whom these things are important.
The Tory MP John Redwood drew howls of disapprobation when, as Welsh Secretary (a satirical appointment if ever there was one) he obviously had not the faintest idea of the words of the anthem in Welsh - he would have been well advised to keep his mouth shut!
Finally, my history teacher (who described himself as a Disraelian Tory) was very fond of quoting the Whig Samuel Johnson that patriotism is the last refuge of the scoundrel. Robert Maxwell was very fond of the Union Jack and was one of the people behind the 'I'm Backing Britain' campaign and what a scoundrel he turned out to be!