Hello, I'm currently on the Level 3 Extended BTEC Diploma in Business Studies and it is the equivalent to 3 A levels. This BTEC gears you towards all aspects of business, but law does come up in multiple units and has its own. As above, A levels are what employers and uni's are familiar with, however this doesn't mean that they can't cater for the individuals who have taken the BTEC.
Realistically, you don't need any previous qualifications to study a law degree, but most uni's will look out for individuals whom have this A level with particularly strong grades. For instance, one uni might look out for A,A,B where as others may look out for lower grades. Also it will help having previous qualifications in law so you have a grasp on the subject before you start the degree. Your spending money on it, so you might as well do something before hand, for free and see if you actually like it first.
As for your GCSE grades, you should be aiming for B's or above to enter into most A levels, but the BTEC is much more lenient and if you have previously done business, which in your case you haven't, it would put you in a much better position, thats if your sure that what you want to do. I got C's and D's in my GCSE exams (D's in the subjects I wanted to do, including business) and I got excepted due to my business coursework being at a B, this is due to the course being 100% coursework at this time. HOWEVER THE SPEC IS CHANGING NEXT YEAR TO 60% COURSEWORK, 40% EXAM, KEEP THIS IN MIND.
There's also the option of getting a apprenticeship with a law firm at level 3, then progressing down that route, but that a whole different can of worms there...
Hope this helps!!