The level of English exam you need, in order to get a study visa in the UK, depends on the course you want to study.
To study English in the UK, you need a Short Term Study Visa valid for 6 months or an Extended Short Term Study Visa for 11 months. You are not entitled to work on this visa. It can not be extended in country. You must return to your country of origin to reapply for a visa in order to stay longer.
To study a course below degree level-
You will need to pass an English language test set by a Secure English Language Testing (SELT) provider.
The SELT test will assess your reading, writing, speaking and listening abilities. The level you will be expected pass is CEFR (Common European Framework for Languages) level B1.
To study a course at degree level at a college that is not a Higher Education Institute (HEI)-
You will also need to take a SELT test but you will need to pass CEFR level B2.
To study at degree level at a Higher Education Institute (HEI)-
To study a course at degree level or above at a Higher Education Institute (HEI) you may not need a SELT test. The HEI may have their own method to assess your English language ability. You will need to check with the individual HEI you intend to apply to. However, they will expect your English to be at the equivalent of CEFR level B2.
There are two different test results that are accepted as part of your visa application.
It is important you only take a test on the SELT list and book it at an approved centre. The UK government website has a list of approved test centres.