I'm not sure about allotments, they don't seem to work that well to me but it's just anecdotal knowledge. My dad started sharing one two years ago so I went around a few times. More than half the plots were completely unused. A few plots were very used but felt like a private place to escape. I looked up the waiting list and it was over 60. They said if you don't use them you lose them but it didn't seem to be enforced at all. Anybody not involved in an existing one was not allowed or welcome in.
Surely it would be better to have lots of community gardens run by the same people who have the allotments but where anybody can take part and local people are encouraged to help. Get school groups involved. I appreciate it's a lot of effort but otherwise, does it really matter.