Apart from the insurance issue, there are two other considerations
who owns the land on which it will be built?.
If it a public park the ownership will vest in the Local Auth
What happens if the RA folds up due to lack of funds?
You would need a more robust legal entity than an ad-hoc RA to run with this.
Did u talk to the Local Auth?
The insurance would work on the basis of max pay-outs for claims which raises the issue what happens if the claim exceeds the policy payment amt, hence the need for a robust entity.
In the extreme case if the directors of the robust legal entity were found to be negligent then the claim could pass through to them personally