I think if you can hold them back, you should also be able to push them forward. If not, the cut off should be the cut off and that's it.
My child is going to be the oldest in her year group by far. She misses the cut off by ONE DAY. This is a child who is more than ready for Reception, more so that many of the children who are only weeks older than her and moving up whilst she's still got another year at playgroup to go. She'll already be 5 when she starts Reception next year.
Mine is a second child, she regularly plays with children older than her very happily. She's already writing her name, ours and other words, doing simple sums, and blending letters to read simple words. She joins in with her older sisters spelling practice from school, asking me to make her her own version (only simple worlds like Bat, Cat, Mat etc). She can use the toilet, change her own clothes/shoes, do things like open a yoghurt independently, knows how to follow the routine at playgroup/sit on the carpet nicely/raise her hand for things etc and has no problem mixing with a range of children. She's very confident.
I can't move her forwards. She has another year at playgroup which she attends three days a week and she is going to be so frustrated this coming school year. She's cross because her friends are moving to reception without her and she's being left at playgroup with children some of whom are nearly a year younger than her which at this age is a big gap. Some of them are only just turning 3 and are still in nappies.
I'm not fussed from a childcare perspective either - I'm a SAHM so it means I get an extra year having some time to do nice things with her solo on the days she's at home which is a bonus. I'm not putting her into playgroup an extra day, she's going to start getting bored there as it is, as good as they are! So I've found a forest school to take her to (with me) one day a week to give her some extra things to do and more socialising opportunities.
Before I had a child who was as capable as her I'd have said I'd always prefer for mine to be the oldest in the year rather than the youngest. Now, if I had a choice - for her, not me - I'd send her to Reception in September if I could. She'd benefit far more from starting this year than spending another year at playgroup.
It's not on that it doesn't work both ways! When I was a child there were three intakes a year, my brother started school the January after he turned 5 at the Christmas. Worked better!