Hey and welcome to the hobby! While I'm not in the
UK I can at least give a teensy bit of help, I hope.
One of the best things about joining Warhamer now is that
AOS is new for everyone so you won't find anyone with disproportionate years of knowledge, even though they may be players for 30 years, since we are on the starting phase since the game is only 6 months old
If anything, newcomers have less Fantasy to unlearn--I mean that in a fun way, like what I'm trying to shift gears in my brain from dwarf to duardin.

I had these same questions 4 years ago before my first tournament and found the experience quite enjoyable. Smaller events are good for socializing and making new friends, especially if there are food options to keep the players together or on site. From my understanding of chatting with some
UK tourney movers and shakers it's not that different from the US. You still have soft scores, even if they are not weighed the same. That means sportsmanship, painting (you guys recently added display boards) and sometimes a scoring for fluff lists over win at all cost optimized lists. I've never heard of a tournament in any game where new players are not welcomed and helped learn the event side of the hobby (at appropriate times of course). If it's a large event you may find a split after the second game where the higher scoring players compete for top spots and the rest of us know we are not getting anything so have fun on the bottom table area, you can even set up camp and make yourself comfortable

But certainly make sure you are having fun.
Try telling the event organizer ahead of time that you are new and they might /should pair you with an appropriate first round opponent. I don't see why European events wouldn't do that. And personally, I wouldn't worrying about what's the best list or what the Internetzzz commands you take, especially, again, with
AoS being so new. Play what you like, even better if it allows for a cool narrative to give yourself and your opponent a great story (making new friends) so that you both have fun and enjoy. You won't find a room of players only wanting to win and damned be the rest...especially with
AoS. If the event has a social Media page post something before the event to introduce yourself and put some names with faces if you can. You should absolutely walk away from the event with new playing tips, new friends and great stories. Best of luck!