You will usually need a 2:1 degree from a UK University or equivalent.
If English isn't your first language, you'll need to show evidence of your English language ability. The minimum IELTS test score for entry to a research programme is 6.5 overall, with at least 5.5 in each element of the test. However, some postgraduate research degree programmes are linguistically more demanding, and higher command of the English language and/or higher IELTS scores are required. This will be indicated upon advertisement of that specific research degree study opportunity, upon contact and/or following interview with the applicant.