I'm going to be studying for the Double Maîtrise course at King's College, London starting this September. On the Open Day the prospective students for this course were told that indeed a high level of French is required - certainly A at A Level or equivalent, but most of the students on the course were by no means incredibly fluent, they just worked hard at the language side of the course as well as the Law. At KCL I think extra classes in French are offered to the English speaking students and all the French Law classes in London are taught in French. They also take into account everyone's different circumstances - say for example one student's mother is French whereas the next has never been to France before - obviously they would be expecting a higher degree of fluency from the first and would take everyone's individual circumstances into account.
I am good at French, I got an A in my Advanced Higher, but I am no means perfect. However, I'll be working my hardest to achieve the best possible French I can when I am in London and supplementing that with work experience in France and trips over there whenever I can. It's going to be a very difficult course to do - but well worth it - the consequences of completion of this course look very good indeed for the students.
I and legalbeagle (who is waiting on results at the moment, but I'm sure will achieve well!) and Liam52 on these boards should all be studying Law and French Law at KCL, the LSE and UCL respectively in the Autumn, so if you have any questions, just post and I'm sure someone will be able to answer.