Santa - one of the best things about th UK system is that instead of holding kids back we differentiate and try and teach all the kids in the class at the right level for every subject.
I really wanted DD to repeat reception, and of course she couldn't.
Now she's in Y4 I'm so glad I didn't keep her back. She's still bottom on the class. Still very happy and loves school. Still being taught at her level and still got friends.
If she had been kept back she'd still be bottom of the class but would probably find the other kids in her class immature and have self esteem issues about being kept back and still not being able to keep up.
I can't think of any good reasons to keep children back who have 'failed' a year.
See that's why the school system here is so good. Nobody fails.
I really don't understand what your problem with reception is. I don't think it's all that different from nursery in most other countries. Kids certainly don't have to write. My DS never wrote at all. I got his literacy book home at the end of the year and it had something written on one page. I wasn't aware that he was meant to have been writing. He was happy.
He didn't like Y1 when he had to write. But he was 6, so not young. Thing is it turned out the reason he didn't like writing was nothing to do with immaturity it was to do with SN. Doesn't matter how old he was when he started school. He was going to hate writing.
And equally loads of kids don't learn to read in reception. They learn to sing phonics song, same as they learn wheels on the bus and twinkle twinkle. But some of them don't learn to read and it's not a problem.
And if you're worried about them missing out socially - well that is the main point of school. That is why kids go to school. So that is why you should send your 4 year old.
I really don't know why you don't want your 4 year old playing with other 4 year olds. In the vast, vast majority of cases they love it and progress well.
And of the tiny minority who don't like it, not all of them would have been ready for school at any age.
Some of them will struggle due to undiagnosed SN, and if they'd started school when older their SN would have stayed undiagnosed ( and unsupported) for longer.
Some of them will just struggle because they're lacking the social skills to survive at school. And even at 7 or 8 they'd still have poor social skills and struggle.
Honestly, school in each country means different things. Make sure you really understand reception before you slag it off.
And quite often it's the parent who struggles with sending their baby to school - not the child at all. It's the parent who doesn't want their child to grow up and do all the things a 4 year old should do.