is unintentional weight loss most likely to be cancer?

Long time no see. If you are worried about cancer (which you are statistically unlikely to have, considering your age, but no one on the internet can tell you), TSR is not the place to come and you should see a doctor.

I've seen a doctor and they bas8vally just tell me to go away lmao

Well in that case it t sounds like you don’t have cancer, and asking unqualified folk who have never met you isn’t going to lead to a more accurate diagnosis.
I seem to remember that you were worried about a brain tumour or somesuch not so long ago. That would suggest that either your general health is poor, (leading to a lot of a different symptoms), or you are just very, very worried about your health in general.

My health isn't poor but I always have the symptoms I am reporting, and doctors often send me away refusing to do any tests at all/insisting they aren't needed which doesn't put my mind at ease at all. It takes weeks of booking every appointment I can to even get standard blood tests, and I have requested MRIs but always just get told they're not willing to refer me for one on the NHS and I can't afford to get tests done privately. I have also asked if there's anything they are willing to prescribe me for the persistent symptoms and they always refuse. Some issues I have been seeing doctors about since I was around 12-13 (8-9 years) and have made no progress due to doctors thinking I'm either making stuff up for some weird reason or just believing it's acceptable and normal to expect me to live with near constant and sometimes debilitating pain.

My health isn't poor but I always have the symptoms I am reporting, and doctors often send me away refusing to do any tests at all/insisting they aren't needed which doesn't put my mind at ease at all. It takes weeks of booking every appointment I can to even get standard blood tests, and I have requested MRIs but always just get told they're not willing to refer me for one on the NHS and I can't afford to get tests done privately. I have also asked if there's anything they are willing to prescribe me for the persistent symptoms and they always refuse. Some issues I have been seeing doctors about since I was around 12-13 (8-9 years) and have made no progress due to doctors thinking I'm either making stuff up for some weird reason or just believing it's acceptable and normal to expect me to live with near constant and sometimes debilitating pain.

How many medical appointments do you have a year?

Debilitating pain for over a decade is poor health.

about 5 for each issue, then I give up for a while when they continuously refuse to help me

How many issues? How many appointments in total?

I'm not doing anything to cause my health to be poor though, I have a relatively healthy lifestyle and have followed all advice doctors have given me

To echo another comment above: A forum such as this with students in is not your best source of medical information.
There is a multitude of reasons for some of the symptoms you’ve talked about, not necessarily always cancer.
If a qualified doctor has told you that your condition is not of concern, then their judgement far outweighs any suggestions here.
If you have only seen one doctor for all these appointments you can ask to speak to another impartial doctor for a second opinion - but I gather the impression this is multiple doctors telling you the same things here.
Having persistent anxiety over your health is a medical condition. So if you find yourself concerned about your health almost every day, then that is an anxiety you should seek help for.
I’ll ask you a final question, with the number of appointments you’ve attended, which do you want more - a diagnosis, or symptom management?
If it’s a diagnosis you want, then are you going in with the wrong priorities?
If it’s symptom management you are looking for then it’s better to approach the situation as “I have symptom X, and want some advice”, rather than “I have cancer, I want you to confirm that for me”.

Except not eating enough, drinking unhealthy amounts of alcohol and taking opiates, which you have been told multiple times are only making your issues worse.

Alcohol and opiates weren't mentioned here and aren't a part of this issue. Other issues such as the migraines I've had since I was a child.

But they are contributing to your poor health whether you like it or not.

If my other health issues were actually being treated then I would be able to stop the drinking and painkillers

If my other health issues were actually being treated then I would be able to stop the drinking and painkillers

you claim to drink because you enjoy it