Studying a language in a country where it is widely spoken has many advantages. It is, therefore, a good idea to study English in a country such as Britain. However, I believe it is not the only way to learn the language.
In the first place, most students in non-English-speaking countries learn English at secondary school, and sometimes at university nowadays. Although their spoken English is not usually of a very high standard, their knowledge of grammar is often quite advanced. This is certainly useful when students come to an English-speaking country to perfect the language.
Secondly, studying the basics of English at secondary school is less stressful than learning the language while overseas. This is because students living at home do not have to worry about problems such as finding accommodation, paying for their study and living costs, and trying to survive in a foreign country where day to day living causes much stress.
However, there are obvious advantages of learning English in Britain. Every day there are opportunities to practise listening to and speaking with British people. Also, students can experience the culture first-hand, which is a great help when trying to understand the language. This is especially true if they choose to live with a British family, as exchange students for example. Furthermore, if students attend a language school full-time, the teachers will be native speakers. In this case, not only will students' speaking and listening skills improve, but attention can be given to developing reading and writing skills as well.
In general, even though it is preferable to study English in an English-speaking country, a reasonable level of English can be achieved in one's own country, if a student is gifted and dedicated to study.