Law is a VERY academic subject, and it is VERY competitive.
You will need 3 good A levels (A or A* grade) to even be in the running for Law at a RG Uni - because that is what everyone else applying will have.
A BTEC in a weak subject like Business? Not a hope.
You may be able to apply for a Foundation course in Law - but again these may insist on academic A levels as well as a BTEC.