As per MusiCol's opening (edited) post, please can those interested in attending list any gear they're willing to bring along? Other than having enough sponsors with lots of gear to look at, it's people willing to bring along their own gear which will make the meet interesting and a success (plus I would say UK based sponsors are less likely to have any of the more exotic/further afield gear).
Here's my modest list of stuff I will bring:
Headphones
Sennheiser HD 25-1 II
Sennheiser HD 650
Westone UM3X
Amplification
Little Dot I+ Hybrid
Meier Audio CORDA STEPDANCE
Little Dot MK IV SE
DAC
Nuforce uDac 2
Little Dot DAC_I
A fairly meager and not uncommon selection I know - please help us build on this!
For example, it'd be great to see any amps by Woo, DIY (i.e. AMB M^3 and B22), the Senn HD800, Beyer T1, any Orthodynamics and an Audio-Gd DAC or two... (Not that I just reeled off my personal wish list or anything...!
).
It would also be really useful to get a list of UK suppliers for us to approach. The ones I know of off the top off my head are (at least I think they're all UK based):
http://www.hifiheadphones.co.uk/
http://www.iheadphones.co.uk/
http://www.ampcity.co.uk/ (these guys have got some interesting gear)
They don't necessarily have to be dedicated headphone suppliers if they've got some interesting source gear, etc.
i.e. http://www.audioaffair.co.uk/ has a nice headphone section on their website.
Then of course there are the UK based amp makers such as Graham Slee, etc.
Lastly we might be able to get one or two of the UK headphone manufacturer reps. to come along, Sennheiser, etc.
Bit of a rambling/unfocused post I know, but if you can help us build a list of gear that'll be at the event and potential suppliers to approach, that should really help peak peoples' interest and generate some real momentum.
To further echo MusiCol, there's still the small matter of agreeing a date, location and venue, so please keep posting your views on this! 
Lastly if anyone is willing to dedicate some time to help us organise this, so we can divy up what will be a large to-do list, that would be fantastic and would significantly increase the chances of this going ahead and being a success.
Many Thanks.
I'm going to do some reading up on past events and get in touch with previous organisers to see what I can learn and add to the good start MusiCol has made on listing everything we need to cover off and will post back soon.