what are you saying? Further Maths has nothing to do with Law. Same applies to Chem. there is no "must". You are living with your decision but it does not imply others need to follow.

short answer - science stream, you need to justify your ability and interest in Law

arts stream, you need to compete with people doing the same type of subjects so you won't stand out... or worse, people with more impressive subjects (Maths Lit History Psych Econ other combos not available in Malaysia)

Well of course there are science stream students who are doing Law degrees The guy that came to my college to share experience on Oxford Law admissions did science stream. But there are many other traditional A-level subjects you can take instead of further maths. It sounds noble of course, doing it for critical thinking, but FM just sound a bit outlandish for me because most of my friends who took FM want to do engineering or mathematics or econs, or just like maths a lot and are reallyyyy good at it. And some of them actually like doing math during their free time.

Are you only taking 2 A-level subjects, Chem and FM? =/

There's actually no need to take Further Maths. Even students who opt for engineering don't need to have further maths in their A Levels (Maths itself is already very challenging). However you can go for science stream in A Levels in case you change your mind and don't want to take up law in the future. (A Levels is an eye opener on how in depth a certain subject can be... so yeah)

There are no fixed subjects to take during A Levels if you intend to pursue a Law degree in university. You may choose to do Science subjects, or take Arts subjects, but the general trend for students (as far as i see) is to take Arts subjects, because:

1. they're more relevant
2. the subjects are more towards subjective thinking, crictical thinking and reasoning which will prepare you better for the Law degree you intend to pursue.
3. they don't like sciences 

Anyway, no harm doing Science subjects, but you'll have to justify how these subjects prepared you for your Law degree in your personal statement. At the same time, do check out the requirements on university websites which you wish to apply to see whether they may require you to take arts subjects.

IMHO, Further Math may be able to revv up your logical thinking and since it's a traditional subject it is weighted more heavily than law and accounting. But I don't see how subjects like chem and biology would be relevant to your aspirations (unless you're aiming to specialize in niche derivations biotech law, which you should duly explain in your UCAS statement...).

I agree with everything @Flame Haze has said. You should take up subjects that improve your writing faculty. Law as an a level subjects is a bit of a grey area; it's not a prerequisite at many universities because your first semester and parts of your second semester will be a complete rehash of your A levels law syllabus. Many of my friends who took up law as an alevels subject and proceeded to read law as a degree felt that their first year was a waste of time.

Subjects like economics, business studies etc overlap a little bit with law. A large chunk of your syllabus will deal with company and commercial law etc.

Try to think about this : What type of works you preferred in the future? Office work? Work in hospital / lab / construction site?
If you are able to answer this question, I guess you know what you want and choose the correct pathway