Yep I have two law degrees, an LLB and LLM and I am currently studying for the entrance exams to the law society, so suffice it to say I am nearly a pro at doing law exams now...
As you say yourself, a lot of it is down to memory. I find half the battle is remembering the sheer volume of cases and the principles that go with it (the other half is being able to intelligently apply those principles to the question asked - law examiners HATE people who write 'all you know about X' type answers).
I find as part of my revision I just write out a big list of all the important cases for each topic, and a word or two of the principles that those cases established beside each. Eventually I will start to just write out the list of cases, and I will find that I will have remembered the relevant principle for each without needing to write it down. I will look over these lists religiously before the exam, and as soon as I am inside the exam and given any piece of paper that I can write on, I will write down as much of the lists as I can remember. Once I have the cases down on paper, the principles from each will come to me automatically, and I will be able to formulate a good answer based on this.
One tip, very little marks if any are awarded for remembering the exact dates of cases. In fact in my experience you don't even need to put down the full name of the case, a lot of the time just the name of the plaintiff will suffice. And even if you're in the exam and you remember the principle established by a certain case, but you just can't remember the name of the case, writing down the principle anyway will still yield good marks.
As for doing an LLB at the same time as a maths degree...are you some sort of masochist?!