unhappily, i was diagnosed with arthritis at quite an old age (60+) after happily eating tomatoes and tatties for all my life. i won't say what i think about advice like this.
Assuming so, something that might be worth considering is medical cannabis, which has been legal in the UK since 2018 - I mention it, as very few people are aware of this. Both THC oil and dried cannabis flower for vaping are available. I use it for chronic neuropathic and muscular pain, and it works amazingly well.
Alternatively, as some others have mentioned, your body may not process codeine correctly (quite a few people can't), so you may wish to try co-dydramol, a mixture of paracetamol and dihydrocodeine. I personally found dihydrocodeine to work much better than codeine.
not a scot (actually i'm english - shock, horror) but i was married to a scottish girl and lived in edinburgh off and on for about 10 years, and some things have stuck.
i think that i am processing codeine ok, as when i took some of my late mum's pure codeine tablets (she got them for awful menstrual/menopause problems) it sent me off into a warm nice place. but try getting them from your gp these days, if you are a man.
Arthritis and inflammation in general can be caused by many things. For some, it can be the good one eats. Nightshades are a real problem for some people.