How bad is it to take 30-50mg of codeine a day

Codeine is addictive. It also has the potential to make your headaches worse over time (i.e. can cause medication overuse headaches)

Research addiction to painkillers

I know it’s addictive but will it actually cause any health problems at such a low dose? I’d stop taking it if it made my headaches worse but I haven’t found that to be the case so far

First of all, if you use it every day you may become further addicted and may experience cravings/desire to take more. Codeine is converted to morphine in the body, morphine can be fatal in high concentration. Over time, your body may become tolerant to the effects of codeine, requiring higher doses to achieve the same level of pain relief, which would further increase addiction.
Codeine can supress breathing which may reduce your overall fitness. Codeine can also cause constipation, nausea, and vomiting. Codeine is converted to morphine in the body, morphine can be fatal in high concentration. You could also face withdrawal symptoms as well which could be worse than the original pain.
This is not to say that codeine should not ever be used when it is needed, it is a good painkiller, when used appropriately.
Overall you should talk to a healthcare professional. They could help you find the root cause of the headaches, so you would not need to use codeine.
You can talk to someone from NHS 111 service. (Call 111 or text https://111.nhs.uk) They can assess your symptoms and provide advice on the best course of action.

I take cocodamol for my migraines/stomach cramps that I get basically every day. I just buy the over the counter ones that are 8mg/500mg and take two tablets two or three times a day (so 32-48mg of codeine a day). I know you're only meant to take it for three days but will it really have any negative impacts on my health if I stick to such a low dose?
I get bad headaches/stomachaches a lot and paracetamol or ibuprofen don't really work for it. I'm also at uni so I can't really have a lot of sick days where I don't get much work done. I've been to the doctors multiple times and their answer is just that some people get more aches and pains than others and I don't have any diagnosable condition, I'm just in pain a lot.

codeine causes constipation and stomach pain most likely your stomach aches are due to that. Further OTC codeine is very weak your better off using the recommended aspirin dose or getting pharmacy medicines

You need to go back to your GP and discuss an ongoing treatment plan.

If you're getting problems with stomach cramps could it be food intolerance? It's not normal to get stomach cramps all the time and it won't necessarily be helped by a healthy diet. Do you notice any pattern with the stomach cramps? Like they happen not long after eating certain meals?
As for the headaches are you drinking enough water and getting enough sleep? Could they be caused by stress?

I’ve been to my GP multiple times and their answer is just that some people get more aches and pains than others and I should just focus on a healthy lifestyle

So you go back to them, explain what you've done with regards to your lifestyle, and that you are still struggling to manage your symptoms.

Going back to the original question.
"Even the use of lower-dose codeine medications over an extended period can cause dependence, problematic use, risk of slowed breathing, and death."
https://www.canada.ca/en/health-canada/services/substance-use/controlled-illegal-drugs/codeine.html
Edit: I'm also surprised that you are obtaining that volume without any questions arising from the pharmacy. There was a recent poster who was getting challenged about this, (and I think may even have been asking about how to get around it), but I am sure that this is quite unrelated.

I have. I’ve stopped bothering now but I’ve been to multiple different doctors (because I moved in the years I’ve been going about this), said my lifestyle is perfect and I take nothing but paracetamol, had blood tests done, had an MRI (because I fainted from how bad the headache was once) and their answer is always just that I’m healthy and there’s no cause