I saw a travel show once and they tried to explain it, it was similar to what was said earlier. Most people dont have pools in the UK, so in the summer if you want to swim you go to the local public pool. And to stop people from wearing street clothes in the pool, they had very strict rules, only skin tight attire was allowed, so either a bikini or one piece for women, and the speedo for Men, no trunks of any type were allowed. And so after years and years of having to wear them you get used to them and that is what you wear whenever you go to a pool regardless of what country you might be swimming in.