I'm a republican and i don't live in the UK but anyway (I am British if that helps?)
I quite like the idea of a bit of a street party or a picnic in the park - i think that some neighbourhoods really got together to be supportive during the lockdowns and it is a shame that things seem to be getting back to insularity, so i hope this brings back some of that togetherness, especially for people who live alone, or don't have many friends etc. Just a day when the children can run around and everyone can sit on a slightly damp field and chomp on an egg & cress sandwich, a droopy sausage roll and drink slightly warm pop/beer from a paper cup. It sounds quite fun, actually. I love bunting (not sure about the Union Jack stuff, but generally bunting is jolly) so all the little pennants flapping in the wind usually makes people smile.
But: extra bank holiday? if you work in an industry that works harder on holidays (chefs, caterers, etc) then it's no holiday and any amount of TOIL doesn't give your kids the day off when you have it. Shiftworkers? do you benefit from 2 extra days off? So basically, it's a 9-5ers holiday/celebration and everyone else gets to carry on as normal and gets to watch everyone else having extra fun. Which is a bit rubbish really.
But. Hopefully this is the monarchy's last "hurrah" and when her Maj pops off, hopefully not before August [for selfish reasons], her very own PFB will be the king that presides over the country becoming a republic. And please no tiresome, lack of creative thinking, cries of "but who wants president Blair/Johnson/Rees-Mogg?!" There are other people who could be a non-exec head of state. Ian MacMillan? Floella Benjamin? Tanni Grey Thompson? Professor Brian Cox? Mary Beard?
In short: have fun if you're joining in, enjoy the days off if you get them and for anyone who is working, i hope you at least get time and a half and a day off in lieu.