Hey there Reddpanda,
I've never been to the UK, but these AB/DL meet-ups and munches are almost everywhere these days. I've been to quite a few in Canada, a couple in the big city of Toronto and a handful of them out on the East Coast, my usual stomping grounds, so I think I can shed some light on what to expect.
From personal experience, almost every munch is different. I've been to some munches that took place at public locations, where people just got together in regular, everyday clothes and chatted about our shared lifestyle. I've been to other munches that took place at bars after hours with the doors locked, these ones permitted wear and play, but the usual rules applied; no unwanted advances, no sexual conduct and no using diapers etc. So, if you can, I suggest you look up the meet-ups or munches you are interested in attending, you'll learn a lot about how they run and operate and what the rules are by doing so. Remember all meet-ups and munches are different.
From my experience, none of the munches or meet-ups I attended were sexualized in any way, there were certainly people in attendance who fall on the more sexual end of the spectrum, but they are counterbalanced by people like myself, who express this as a part of themselves and embrace it as a lifestyle. Furthermore, with all the rules and guidelines in place, these events tend to discourage the sexual element, it's a place for people to gab about their shared interest and make friends not a place for them to explore their deepest fantasies. Munches and meet-ups are basically either super chill venues for people to talk shop about being an AB/DL or they are much more open, playful affairs that encourage wearing and fun party games
As far as rules, they depend on the event. Some of the more public meet-ups and munches of course have a strict no diapers or concealed diapers only policy, discretion is very much the name of the game. At the more open meet-ups, diapers and onesies and accessories are aloud, but there is to be no sexual conduct and the diapers are for show not for use.
Also from experience, atmosphere depends very much on the type of munch or meet-up as well. Also, be forewarned, if you're a newbie, the atmosphere is always going to feel a little awkward and judgy the first time around, attend more than one munch/meet-up and get to know people within your local community and the atmosphere will shift towards an accepting one. Also, the location helps too, I find that it's sometimes better to take advantage of munches and meet-ups that take place in larger cities, there's more people there, you won't be the only newbie and at larger munches, there's always a few nice folks there who show newcomers around. The munches I attended in Toronto were quite fun for this reason alone. On the East Coast, the AB/DL community there is small, the smaller the city and the smaller the community, the more cliquey things can get. My first few munches out on the East Coast were extremely awkward as I was the first newcomer in some time and because everyone had their own clique, no one was really open to taking me by the hand and showing me around. After a few munches out East though, I got more comfortable introducing myself and I actually have a few AB and DL friends out east that I correspond with now
Oh and one last thing, remember to have fun, if you can manage a conversation or a new friend at your first munch/meet-up then you're doing fine
Hopefully, that answered some of your questions.
