I think that the people from My Society (UK org) can help in thinking about org structure. They are based in the UK but mainly operate globally with their own international arm as well as third parties. Will try to get in contact with Tom Steinberg (Director), if anyone has a direct contact with him, please let me know!
The people form NESTA in the UK can help with advice on this issue.
Another alternative could be B Corps (Benefit corporation) where the economic return is basically a means to achieve a social or environmental objetive, rather than an objetive in itself. This is actually a type of legal entity in the US, I dont think this option exists in the UK, will research. In Argentina, an enterprise can be certified as a B corp given certain conditions that are periodically reviewed (controlled). Also interesting because part of the idea is that consumers (cityzens) can express objections if they feel that a B corp is actually not acting as one. Should see the details about this but the concept is the same as whistle blowing that was mentioned somewhere else. Interesting because this sets it appart from other type of certifications.
Also, there are some organizations which are build as “umbrella” organizations to provide the legal framework that new groups dont have. Res non verba in Argentina is an example of this. They provide legal and accounting services, and try to promote similar projects so that complement/ help each other (charge a % of donations received). Maybe something like this in the UK? The idea is that after the initial stage, these groups develop their own legal entity once they are more settled and developed, avoiding wasting time and money initially when so many things can change.
And finally, could be good to study in more details the Open Knowlege Foundation and how they organize. Here is a link to the different parts of their community (local groups are related but informal, and chapters are formally part of OKF) http://wiki.okfn.org/Community