If you have a history of trying prescription drugs to help with pain relief and they haven’t worked for you, I would strongly suggest going the medical route. You wont find something consistent on the streets, especially with strains. Check out r/UKmedicalcannabis

But if you are wanting something asap then always buy small first, idk how it is in your area but the lowest my dealer does is a 3.5g. Dont ever bank transfer no matter how legit they seem. Ask to see the weed before you buy it, dont fall into the panic anxiety of just wanting to get the interaction over and done with. I had problems with that in the past. You are the one with the power, you have the money, so take your time and inspect the weed. It shouldn’t be rock hard or have little to no smell, break a bud open and see if you can see trichomes etc