This is an interesting idea...I'm based in Manchester so of course I think its interesting

As already outlined, I'm not too sure I'd feel comfortable working as a subby without knowing that the main contractor was ultra rock solid (financially).
'Buddying' could be a really good idea but again, as already mentioned I think it would probably cost a few quid to set up properly.
Co-Op and Spar shops do something along these lines.
Running it something along the lines of a Co-Op? All members working together towards a single aim.
It would probably require a fair degree of input to set it up and a fair few quid in solictors fees. It would probably require a Board of Directors (or something similar) to resolve disputes and tender for work as a single entity and a Treasurer to look after the finances....no offence but I dont anyone wandering off into the sunset with
MY money!!
Any ideas who would take on these roles? And how much the membership would have to pay for their services?
It would need to file accounts each year (as a seperate company)
It is totally feasible and could challenge some very BIG players...but it would have to be right to get respect from clients and members.
I'm interested

If its just a 'commision based' thing for finding me work that is part of an overall account then you're probably best just to offer it out to tender and that way you can keep the contract to yourself by employing subbies.
I wouldnt be interested.