Not a native Brit -- I don't think Britain is exceptional in this, it's neither very friendly or so unfriendly to children. My kids at their ages are generally free to go where they like and are mostly safe to do so, that's child friendly to me.
Defining being friendly to children by whether strangers give them attention especially compared to animals is a little odd to me, there seems to be a lot of other ways to do that which others have pointed out, but I don't think Brits are exceptionally lacking in this. Mine all got that - some of which was nice, some of which was a pain in the ass and I'm glad they've generally aged out of that.
Honestly, I've no desire to coo at either a baby or a puppy, but then I've had the overstimulated kids who just wanted to quiet for a bit while out who got tired by strangers talking and patting them or the non-verbal child who couldn't perform the basic question and answers strangers have wanted.
they are expected to behave to be convenient to others
I think in the UK, and in many other countries, this is an expectation on everyone, not just children. Not everyone follows that obviously, or we'd have fewer threads on it, but generally the culture ideal is around being convenient, for better and worse. Kids are generally raised with the cultural expectations around them.
THERE IS PLENTY OF JUDGMENT BUT NO HELP.
What help do you want? I agree there could be more socially, but it's hard with the ideal of convenience and privacy to really get that going.
I've had plenty of judgement - I'm a disabled immigrant who was a young mother when my first was born. I've had my less than kind people, but I've also had my very sweet people. Wherever, there will be people who run the whole spectrum. I don't think Brits are exceptional in this.
Honestly, coming from a place often spoken of as "family friendly", a lot of it smiles to your face and talking about you behind your back while absolutely never intervening when there is an issue because 'family is important/family knows best' even for abuse. I fucking hated that growing up al the smiles and hugs and remarks, but ten no actual fucking help for kids in need. I know the UK isn't perfect in this, but as pp said, plenty of other countries have far worse rates of parents killing their kids many known for how much they fuss kids, and having survived an attempt as a child, I'll take one of the chillier Brits snide remarks I got when mine were little (even if not as helpful as the speaker might have thought even when I was fucking up) any day over fussing that doesn't have any real support for kids behind it.