Can I ask, are you taking it because you have a diagnosed pain condition, or are you taking it because you’re abusing it?
If it’s the first, I think you need to have a conversation with the doctor who is overseeing your pain management. If it’s the second, unless it’s an urgent referral to explore possible cancer rather than a routine colonoscopy, I think you may need to seek support to deal with this first and push back the colonoscopy. That is an enormous dose of paracetamol and codeine to be taking on a daily basis.
To give you an idea though, I was previously on cocodamol and oramorph plus pregabalin for cancer pain. I came off them by tapering the pregabalin 25mg a week (was on 100mg). The cocodamol (normal 8mg/250mg one) I tapered by one dose every few days and replaced it with paracetamol. So was taking it 3x a day, so went to 2x a day plus the paracetamol. Then I switched another dose to just paracetamol. Then I eventually got rid of the afternoon paracetamol and then a few days later the morning one. I kept the oramorph to top up whenever I did feel pain but didn’t want to take a full dose. Eventually I didn’t need that either.
Personally, I think you are going to need a supported detox here, not necessarily medicated, but someone needs to be monitoring you and supporting you. I say this with no judgement - I’m a recovering alcoholic. I used to drink over 200 units a week. But quitting something your body is physiologically dependent on and staying off requires more than just safely coming off whatever you’re taking. If you don’t want to go to the GP with this (I didn’t), find a peer support group. Look on Facebook for NA or something similar (it doesn’t have to be NA and you don’t have to engage with the philosophy of any of it), but do it with support from people who’ve done it before. Probably not MN. (Sorry, MN! 😂).