From what I've heard, Legal Studies is merely a sound intro to Law, certainly not a prerequisite as you rightly point out.
If you're looking to do Law at university I would strongly suggest bettering your English mark. Although I see no reason to lump additional pressure by trying to increase a lacklustre Legal Studies mark, improving English would better position you to excel in a Law-related course where essay writing is essential. Now, as I did Legal Studies I can tell you it does take a bit of work, mostly in keeping up with changes to legislation and the like. Can't comment on Chemistry myself but have heard from some of my friends how difficult it was; only 1 person at my school this year got a Band 6 for it.
Ultimately though the choice is yours. But remember, stick to what you enjoy. There ain't no point in staying with something just because you think it might scale well.