My own view on Wikimania 2014 is that it should be set up as a separate
company, with WMUK as a corporate member on the board. Thought needs to go
into what is the best form for it: e.g. Community Interest Company might
be the most appropriate. This relates to how it is going to be funded.

Really that is the only conceivable option. Not only is it far and away standard practice (from a risk, organisational and reputational viewpoint), at some point the Wikimania team is likely to need to employ staff (as other bids have done) and that would likely not be appropriate within the WMUK structure.

Tom