Yeah, it is definitely harder as an adult. I'm not really at a stage in my life at the moment where I'm looking to make new friends (I feel like I'm just barely staying afloat with life and keeping up with my existing friends), but even still I've struggled with it over the years.

My brother actually moved to England with his family for work and had a very similar experience as you. He said that everyone was very friendly, but no one wanted to actually engage in any meaningful way. After two years and essentially zero friends they came back. He's from New England in the USA, which is kind of known for being standoffish, but even still he was shocked by the experience.

I do think the best way these days is to look for Meetups. People that go to them are generally all specifically looking to meet people which helps get over the awkwardness of actually meeting and talking to strangers.