The score required depends on whether you are applying for a taught or research programme.
For general advice and Tier 4 Guidance see here.
The IELTS acceptable score/levels can be found here under the heading Faculty of Humanities and Social Sciences > Centre for Commercial Law Studies- Postgraduate Law Taught programmes OR Postgraduate Law Research programmes.
Also further down that page you will find the other qualifications that QMUL accept at the relevant level to meet the English language requirements. Qualifications used to satisfy the English language requirements should have been achieved no more than 5 years from the start date of the course of study.
You will also see Postgraduate guidelines at the bottom of the page which will be useful for those students who have taken a degree from a majority English speaking country and where exemptions may apply.