It is an interesting point.
You could certainly threaten the proceedings but I’m not convinced they would succeed in court. It would depend whether the neighbour had done this simply to be a pain in all your sides whether he has a genuine reason for objecting.
There is a legal test with regard to any intervention like this called the “but for” test. For something happening but for someone doing for someone not doing something, XYZ would not have happened. You could argue that but for his objection you would not have been flooded because the flood alleviation scheme would have gone ahead.
You can tell him in your legal threat that if the properties are flooded as a result of the scheme failing to go ahead because of his objection, you would be suing him for the cost of the remediation. That kind of lawsuit would probably lead to his bankruptcy if it went ahead assuming of course that he could fund it in the interim.
Unless he is a multimillionaire, seven against one are not good financial odds even if the claim is not a particularly good claim.
You could do that now and say exactly that. If there are 7 families involved, the legal costs divided by seven is going to be far more attractive the legal costs paid by one! However any family that doesn’t join in any potential litigation is still going to get the benefit of any positive outcome without having the burden of a negative outcome.