What to do with research results while being in high school?
First, if it is not submission-ready, then it is unlikely that it is arXiv-ready either.

If you put it in another place, you won't get much prestige or recognition, so I would aim at a place accessible to everyone (without login) - your homepage, GitHub or anything (it does not matter). It may be a good place if you want to start discussions.

Second, it is rather unlikely that it's something ground-breaking that everyone would like to steal from you. (But don't get discouraged -  it is certainly possible that it is an interesting result!) You would benefit more from discussions than have to lose.

Third, some guidance is needed. Ideally, you can consult it with a friendly teacher, professor or PhD student. If not possible, one way to go is to try some research-level competitions for high school students (e.g. like First Step to Nobel Prize in Physics, European Union For Young Scientists or something in that line). Even if you don't win anything (those are very though competitions) you might get some feedback. Additionally, even in Poland there are a few local competitions for works in mathematics by high schools students (e.g. this and that). I am sure in UK there are also some. If you did something in a Math Olympiad, you can try asking organizers if they know such competitions (or someone suitable for mentoring you).