Moving to the UK - medical needs (Johannesburg)

I will be emigrating to the UK next year. I am on a lot of chronic medications and I have extreme joint and back pain for which I am taking very strong pain killers (opioids). I have heard from many sources that I will need to go through my UK GP for all my medical requirements and that these GPs are extremely reluctant to prescribe strong pain killers and I've also heard some horror stories about people with depression not being able to get the anti-depressants that they require. Can anyone suggest how I can find out and ensure that I will get the medication I need on the NHS when I move there? My whole moves depends on this because I am too scared to go unless I am assured that I will get my medicines.
Thanks in advance.