In order for you to enrol in any of the UK universities or colleges, you should have a proof of English language proficiency, unless you are a native English speaker.
IELTS or The International English Language Testing System is a language proficiency test for non-native English speakers. This is the only test accepted by most Australian, British, Canadian and US schools and universities.
In the old days prep schools used to be small institutions for preparing boys of 8 to 13 years old for boarding schools. Today, most of the schools are coeducational and many offer pre-prep and nursery.