Mostly language barrier combined with social anxiety. I had two bad experiences in Europe that give me cause for concern. The first was immediately upon landing in the UK where some awful combination of jetlag/culture shock/inexperience led me to sulk in my flat for almost two days, and later when I was robbed in a hostel in Copenhagen. The problem in both situations was that I was petrified to seek out help, even though, in the back of my mind, I knew that someone could help me. It's been about two years since then and I think I've grown out of much of that excessive introversion, but part of me still worries about stuff like that when visiting new places.
It's good to hear your positive experiences, I suppose I'll just need to be a little more courageous than usual.
All of that besides the language barrier just sounds like a mental thing, like you're imagining people to be more different than they really are.
As for the language barrier, Japanese are typically terrible at speaking English, but of course they know very well what "thank you", "please" and "sorry" means, and that's really all you need. Public transportation is typically in both English and Japanese, and menus are often in English too, and if they're not, it doesn't matter as Japanese menus usually have pictures on them anyway. And at the end of the day, body language is all you need to survive in any country, no matter where you go.
Mostly because I was young and naive

It was simultaneously my first time abroad and traveling alone plus had quite a bit of trouble finding my flat, so I think that contributed to it. Once my flatmates arrived the next day I realized there really wasn't anything to fear and I acclimated to everything faster than most them did, it was just the initial burst of overwhelming freedom was too much for me. I suppose calling it a nervous breakdown is probably a more accurate description, with cultural differences being but a small part.
Also do cat cafes live up to the hype or should I just spend more time at my local shelter?
He's asking what exactly in the UK was a culture shock to you. I think you're using that word a bit wrong; you're talking more about a general form of anxiety. Nothing here sounds like a culture shock, really
Cat cafés are just strange and awkward in my personal opinion, I tried it once but it's really not my thing, it totally depends on your interests, I guess. I don't know about other cities, but in Tokyo, a lot of people living in apartments aren't allowed to own pets, which is one reason why cat cafés are popular.