Mostly because it's fun. We're a very nihilistic little island so we tend to do drugs for the laugh when we're young, stop when we're about twenty-ish and then pretend we never did it when we have kids.
I did it because my friend pressured me into trying an inconspicuous white powder he ordered on the internet. Research chems made it really accessible to do drugs, and technically it wasn't amphetamines I was doing it was the equivalent of aldi brand meth instead lole
Once they banned research chems I did start doin amphetamine, but I think I'm in the minority of people that did drugs in that I started on the hard stuff and slowly worked my way down to weed nd stuff.
Also I think the sesh culture is much bigger in UK like here ketamine is only considered more hardcore than mdma by like.. idk even who would consider it like that
It's more of a casual thing