Everyone hosting these faces unsurety. OTOH, there's a half dozen or so in the US, now, where 5 years ago, there were 2 or 3. Gotta start somewhere. I help one there and got, what was it, 8 or so people. Seen several of them since. Two of them at the last NEST, and it was FINE seein' 'em both.
How many? If you screen everyone before sending a list, and get rsvps from 'em, you'll have a good idea of the numbers. A munch, weeks before, can help to assure that some folks are comfortable enough to attend.
As to what there should be at such an event, most often, it's half social and half play-party. So long as you see that there's no non-consentual touching, it's all good.
Beyond location, see that you have enough to drink of NON-ALCOHOLIC BEVERAGES. Booze can wreck an event, and let's face it - who wants to forget any of it? Hydrate your 'lees, and provide fluid enough to keep everyone comfortable.
Beyond that? Bindings. Good, stable, comfortable binding for your bondage pleasure *can* be a biggie. If you don't own bondage gear (and many don't) there's several notions. One of the simplest is to take turns just holding on to wrists and ankles. Another's to use thick padding and a simple bit of rope bondage. Wit' rope, have some sort of clipper to cut it quick in the event of emergency. Your 'lee should feel comfortable and safe. Picture having to flail about in bondage, tickled vigorously, and think of what thin bindings will do to your wrists and ankles, and you'll likely figure out a solution that I've not even thought about. There's many of them.
Entertainment? I eschew it, but many hosts find that folks really wanna see such. Personally, I dig the play way too much to pause for a video preview, and several events have serious stalled 'cause all the men in the room stop to watch TV. We can do that at home, alone. In a room full of players, play!
Aim to provide a safe place to meet and socialize. you don't NEED a bed for tickling, for instance. A chair can work. So can a couch. So can the FLOOR, with a blanket and pillow. It's about comfort for the 'lee, and a set of rules for play. I'll send ya mine.
You're right about it havin' been a long time since the lone UK event occured. I hosted that, and there were two women and (I think) 5 men other than me. There are several good and helpful folk, with experience in the BDSM community, whom I hope will jump to your aid.
Good luck! Talk to ya in mail,
dvnc