Regarding the copping convo, I have been able to ask beggars and they've sorted me out. I can go down the end of my road and know exactly who to ask and I'm in what's actually a relatively nice bit of London. Just follow the etiquette by offering them a tenner for themselves. Don't have valuables on your person just in case. Bring just enough cash for what you want and a cheap burner phone with some credit on a PAYG SIM. Nothing else on you. Don't dress in nice clothes, wear a hoodie and jeans and trainers. Look like what they expect you to look like and they won't think you're police or clock you as not being streetwise or whatever.
I'm far from being a roadman type and I even have anxiety but I've yet to get myself into trouble just asking beggars to hook me up. I will usually take a benzo if I'm asking about for something though.
Sorry mods I hope this doesn't cross any lines, as another member brought up before it's a part of harm reduction to stay safe while buying your drugs so that's what I focused my comment on. Everyone knows you can ask beggars for drugs already hardly news but knowing how to stay safe and knowing how to not look out of place while doing it will hopefully help out the less "streetwise" users.