It may be worth checking with the insurer as to wether the site needs to be "supervised" by a named individual when in use. I'd be surprised if the company would issue a blanket cover for all comers.
If they need named individuals then it would make sense to have a core of users who could act as supervisirs - and due to the locality it would make sense to start with the S Wales group. I know I'd be happy to contribute a share towards the first years insurance, and act as a named person on the policy if needed.
As you said you can put it up at the meet this weekend and see how many takers there are - but I'd be surprised if there werent more than enough volunteers.
I'd certainly travel up for it, I think I'll use the car rather than a boat, would suggest you firm up on the quote first then we can work out how to pay it. I for one think if a meet is organised then we charge sufficient to cover most of the costs. If you charge £10 per person for a 2 day meet you would make a good start.