In my experience, it works best around cock and balls, although some guys apparently use the smaller ones that just go around the base of the shaft, and other guys like to have them around the 'neck' of the balls. Those two methods either are non-workable or downright uncomfortable for me.
So I go with the standard ring around the base of the entire 'package'. And start out with a stretchy ring, to get used to the feeling and how the ring works for you. Metal or steel should come later, as it's more involved, and you really need to know what you're doing if you use a metal or other stiff ring. You'd also want to properly size a metal ring, if and when you get to that. With the stretchy kind you don't have to worry about that. There are also special stretchy devices that squeeze the shaft and push down on the balls, I've never used one, but I've seen them for sale and the reviews seem positive.
As for what rings in general do, they add stiffness, a bit of girth, and can add stamina.
I see them as an accelerator, a booster. My equipment still works without a ring, but the ring quickens that development of the erection and also helps with stamina. And it's enjoyable.
As for availability, I'm not in the UK, so sorry, can't be any help with that.
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