As a British person, I've always wondered what we call tennis shoes. After doing some research, I found out that we typically refer to them as "trainers." This term is used casually in everyday conversations, and it's quite common throughout the UK. I've also noticed that some people might call them "sneakers" too, but "trainers" is definitely the more popular term. So, next time you're in the UK and need a new pair of tennis shoes, just ask for some trainers!