One of America's biggest exports is culture - but primarily in English. I bet the UK knows it.
Not every aspect of american culture makes it overseas and much less does now than used to 20-30 years ago.
I think peptides (and medicating yourself out of a hole in general) are a pretty uniquely american thing.
We're amused by your antics but no one wants to emulate American culture anymore.
heck I know I do this with some american stuff
if I asked 20 of my friends about peptides I'm probably the only one that subscribes to that american leaning, peptide-spammed subreddit.
:-)
Also, I'd add that gym bros definitely have heard of them, especially if they are familiar with steroids.
The GLP-1 drugs are probably the main cause of this. We have this amazing class of compounds that work really well, But are priced pretty high, at least considering lower UK wages.
So there is a big incentive to look at getting these much cheaper from China. You hear about this happening, and then you see the fact that these companies have big catalogues full of other peptides that are a crypto payment and a click away.
And, as you say, the gym performance / sports crowd.
The BBC and the Guardian have covered it fairly neutrally, but the likes of the Daily Mail and gossip mags have been both more frequent and more breathless.