You can covert any degree to a law degree, not just history.
After graduating you take the Graduate Diploma in law (the conversion course) which teaches all the core areas of law (as set my the law society/SRA). You will also be able to take some optional modules.
This is a one year course and will then give you a qualifying qualification that allows you to enter the vocational part of qualification (the LPC for solicitors, BVC for barristers).