Choosing events and progression

That 800 time is really promising for your age and if your lean with muscle but at the same time have natural endurance than the 800 is perfect. Id consider doing cross country in the fall if you wouldn't be training otherwise but make sure your eating enough so you don't lose muscle and for track train with sprinting so you specialize in the 8 but have sprint events like the 4 as your secondary. If you work hard and have the right coaching it wouldn't be surprising for you to become one of the top guys nationally for your age.

You're a member of a club? What do the coaches say?

Seb Coe was a great 800-1500 runner - World Records and Olympic medals. He dropped down to running the 400m for a while at the end of the 1970s to improve his speed which is critical for the 800. He came 2nd in UK Championships in 1979 and then set the 800m WR soon after. You'll be able to build endurance at any point in your life but speed is much harder to attain later on when your body powers down how quickly it recovers.

TLDR; focus on 400 until results show whether you are good enough to be a champion. If not move up to middle distance and smash it. Lydiard said there were probably a lot of 400 runners who could have been great at middle distance if they'd done the endurance work.

In my club there are only 2 groups at the top level (400 and below then 800 and above). Atm I do the 400 and below group and so my coaches thought it would be better for me to do 200 and 400. After looking at our national rankings, my 200 time is much worse than my 400 and 800 and is I'm not sure about that advice.

After looking at a few people who ran ~51s at 16 years old, it seems like quite a few of them were just as good (if not better) at 800m. I do want to train for both events but I dont really see a viable way to do that at the moment.

I'll run time trials when my track opens but pacing for 800 (and 400 seeing as I've barely run it before) may be an issue.

I will ask my coaches for advice on training but I'm not sure how good of a response I will get.

I was planning on focusing on 400 but having 800 as a secondary. The main issue I have rn is probably coaching: my club coaches are decent but there is only a 400 and lower group and an 800 and higher group so it's hard to train for both. After looking at the national ranking in my country, it seems like a lot of the top 16 year olds ran ~51s and sub 2 in the 800 and so it seems viable that I would train for both. The only issue is that i wouldnt understand when to train for each event bu ti will ask my coaches for some help.

400-800 runners are rare. I'd guess that's partly a reflection of how they're coached as shown by the setup in your club. It's really split into sprints and endurance. Finding a coach who can do both is hard especially in Britain where decent coaching is rare enough to begin with.

Depending on where you live, you may have alternative club options in which case don't be scared to look around for one that can help you do both. You need to find an Athletics Club over a Running Club. Hopefully your parents can help and support you with this as I'd say it's a daunting task to do on your own at 15yo.

But start off with chatting to your coaches about it all. Coaches usually love people who are enthusiastic and want to know more rather than those who just turn up and barely interact with them.