If you don’t have the money to finance, for example, underground pipes or other infrastructure repairs, then you almost certainly won’t have the finance to litigate which could easily be into tens of thousands of pounds.
Your remedy would be an application to court for judicial review of this decision to dump responsibility on to you all. Bearing in mind that you only have three months from the decision to make the application as there is a time limit. It would only order (if successful) the local authority to revisit the decision which may end up being the same.
You can only make an application for judicial review on the grounds of irrationality, illegality, or procedural impropriety. They are the only grounds.
If there are 100 of you and you all paid GBP100 each, that would give GBP10,000 to spend on legal costs.
If one of you doesn’t want to risk or spend, they still get the benefit of the outcome.
This isn’t one that I be rushing off to court for judicial review. However there is something you can do and that is that you can make a complaint to the Local Government Ombudsman.
If that fails, then you are faced with putting together an allotment management committee and self managing. The problem is that getting 100 people together to all pulling the same direction is going to be like knitting fog.