I feel it started with Diana's death, we seemed to change as a nation at that point. However, social media has exacerbated "public grieving" greatly, everyone wants to add their two cents worth and sometimes in a "very look at me" kind of way. Example, when Amy Winehouse's death was announced, Kelly Osbourne tweeted something along the lines of "so upset can't breath right now", but had a enough breath to get on her phone, to briefly deflect the attention on to her, she is of course not alone in that sort of announcement and as I said just an example. Sadly George Michael is no longer with us, he was the antithesis to all of that, and I had so much respect for his quiet, anonymous magnanimty, it gave us a glimpse of what a kind soul he was.
Turning to the Queen, I think we should remember she grew up in a time of stoicism she was moved enough to put in a very "soon after" visit to the victims in hospital in the aftermath of the Ariana Grande concert. She is of her time, her life has been constrained by protocols, but that doesn't mean she doesn't care. William and Harry belong to a different tactile age.
Nowadays there are piles of flowers all over the place marking a tragic event, although I'm of the opinion all that's kind of futile, never more so than in the case of Baby P were teddies and other soft toys were added, possibly a cathartic effect for some, but nothing changed the fact that poor little baby died in pain at the hands of people who should have cared for him, so maybe the money would be better spent in donations to RSPCC.
Grenfell Tower was an unprecedented horror and the reaction of public grief by those affected is understandable, as is the feelings for all those who witnessed recent terrorist acts. I did at the time think the heads of state marching through Paris after the Charlie Hebdo attack was gratuitous posteruring.