When you go to the destination country, the document you must have is a visa. The document is indeed important to have for security reasons. So, if your destination country is the UK, you can get a visa by taking a test first at britishlifeskills.com. That way, you will get the visa you need.
However, there are several things that could make your visa rejected, such as
1. When asked about activities in the destination country, you cannot answer
You look really confused when asked by the immigration officer. When you are confused, the officer will suspect that you have other goals and can be negative in the destination country. So, if your interview is unclear, surely your visa will not be approved.
2. You do not make a hotel reservation
officers can worry if you are unable or even do not have a temporary place to stay in the destination country. If you already have a hotel reservation, you will find it easier to obtain a visa.