You place so much emphasis on what you can't do! What can you do ?
From what you've written it seems that you wouldn't really enjoy going to parties because a lot of what goes on at parties is what you've said you can't/don't do. However, despite this you shouldn't be too quick to dismiss parties because you need to remember a few things a) not every single person at the party will be drinking, a lot of them will not care what you are drinking e.g. you could be drinking lemonade and say its vodka or you could just tell them its lemonade and be like 'and yeah wott?
' but anyway why should they care about whether you drink alcohol or not?
b) you don't have to dance. You can just stand them with your drink look cool and chat to people, you could do a bit of swaying from side to side but the think is a lot of pl there are not dancers and cannot dance. You're being too self conscious honestly no one is expecting you to bust moves like MJ. However, I do appreciate some people just hate dancing and I am also one of those pl that's why I'm reminding you there is no obligation to dance.
c) Again who says you have to flirt? You can just speak. If you're standing next to someone you could just start up a conversation or say something. You don't have to go in there all smooth and all like oooo baby
Okay now, I'm going to ask you this : what is the worst that could happen? what are you so scared of ? Is it making a fool of yourself or embarrassing yourself? This is actually quite difficult to do unless you're planning to go to the party naked or something. I know you would be feeling apprehensive because you've never been before but you even say yourself that you're a nice, friendly, genuine guy. What's so bad about you that you feel too inferior to go to a party- the answer is nothing. There will be guys at those parties who are a lot worse than you in terms of looks, personality etc however they will be 'doing their thing' at the party like they are the best thing since slice bread - its just all about how you see yourself and not overthinking things and not being scared of the unknown.
Okay so lets say you don't go to a single party or night out at uni...how can you say you are going to be alone! #massiveassumption. There are so many other ways to meet people: sports, societies, clubs, coursemates. There are loads and loads of different things you can join which will allow you to make friends. I'm just finishing my first year and haven't been to a single party/clubbing etc but I've still managed to meet people and not be a lone. Furthermore, you're a genuine nice guy so you will naturally attract people. There is no reason to feel all glum and be pessimistic saying that you are going to be all alone. If you have this mindset it will increase the chances of loneliness.
If people don't want to know you because you have no night life reputation then it's actually a good thing. Why would you want people around you that use your nightlife reputation to judge whether you are good to hang around with or not? At uni you will see that there are far more people out there. Yes, there will be some that hear 'don't drink' 'dislikes partying' and immediately assume you're boring. However, there will also be smarter people that don't use these things to immediately judge you and actually want to get to know you.
Honestly, you need to stop wasting time trying to be like others and just focus on what you like doing. If you dislike partying then do not force yourself to go but rather finding something you enjoy and use that as a way to make friends. For example, I love sports so invested my time in that.
However, I would also say that its important to try new things. Don't close yourself off and create barriers. If you've never tried something you could at least do it so you can say to yourself 'you know what even though it wasn't my thing I did try it and I did go out of my comfort zone'.
As for the whole prom thing, just like you I didn't go to either. When you're in sixth form you're around people who put a lot of emphasis on these kind of events and it's easy to start feeling that you're weird or don't fit in for not going to prom or not going to parties. I think you just need to stay true to yourself. Don't give into peer pressure of feel like you have to go but at the same time don't exclude yourself and hide away- just do what you enjoy and try to use what you enjoy as a way of meeting new people.