I didn't make it to all of the venues suggested in this thread, but thought I'd give an update in case it is useful for any other organisers. I was looking for a quirky venue for a summer networking event, with space for about 50-70 attendees standing, that had some outside space. I always try and use venues that have comprehensive sustainability/environmental policies.
Disclaimer: I have no affiliation with any of the organisations mentioned below, I'm just an event organiser interested in finding unusual and quirky venues.
Visit Brighton offer a free venue finding service and are super helpful. They like a challenge so if you give them a brief, they will suggest suitable options and a member of their team will take you around the city to see venues if that is what you need.
Contact Emma or Darren through this website www.visitbrighton.com/conferences (tell them that Abena sent you!)
i360 - a little corporate for my client's tastes. It was very foggy in Brighton on Friday and you literally couldn't distinguish between the sky and sea from the i360 (which is on the beach). So you would need a clear day to fully appreciate the amazing effect of this venue.
Oh So Social - great space for 100 - 160 people (standing). It's on the seafront.
Merkaba bar - next to Brighton Dome. Good for corporate parties up to around 100 people standing.
Big Beach Cafe - Owned by Norman Cook aka Fat Boy Slim. Funky beach bar in Hove. It would be nice if you wanted a venue that was a bit out of the way where you could make some noise. It's on the seafront.
Copper Clam - lovely beachfront restaurant with a space that can hold about 60 standing.
Riddle and Finns - a few doors down from Copper Clam, another atmospheric beachfront restaurant with a space that can hold about 60 standing. On a couple of different levels so more stairs to negotiate, which may be an issue for those with accessibility requirements.
Brighton Music Hall - a few doors down from Riddle and Finns. Looks like it will hold about 100. Make an appointment to look around, it was closed when I rocked up since it is still off season in Brighton.
I hope that helps somebody!