That's exactly what it is, a game. Some people do enjoy taking it quite seriously, and that's absolutely fine, but you don't have to take
them seriously, or play the same way. There really are folk (self included) who play in what can only be described as costumes.
And nobbers get filtered out, there's no room for them. As Musica noted, we had a guy at our local site who took it too seriously. Not a bad chap, he just got specially excited at a special event, grabbed another player to get his number and that was it, kicked off site and banned. If you go looking for them, you'll find loads of "AIRSOFTER FLIPS OUT" clickbait videos, but those are mostly in the USA where they do things a bit differently. Likewise the sniper headshot tosh that's going around, this is the exception, not the rule. In general, you'll find people laughing with each other a lot more than shouting at each other.
You should be fine if you've bought from a half decent UK retailer.
0.28g is a good shout for woodland - just check if your site requires you to use biodegradable BBs, and how they want you to demonstrate that. I seem to recall one site saying that they expect you to open a new bottle or bag of BBs each day to try and ensure that you're not filling a bio container with non-bio BBs.
As
@L3wisD noted, many (but not all) sites will want to put some of their 0.2g BBs into your mag for chrono. Ask on the day what they expect. If they chrono with their own BBs, then when you're going to chrono, either bring an empty mid cap, or fill a hi-cap with your BBs, but don't wind it up yet - they'll put their 0.2gs "down the spout", then you wind it up.
Another thing to add to your first day itinerary will be to let them know at sign-in that it's your gun's first day too and ask if you can get it chronoed sooner rather than later. There will generally be a marshal or another player who can dig out a chrono and do that, and help you set up the hop. Really, airsofters love to help out.