I wouldn't recommend taking further maths at A-level unless you're wanting to go into a science/mathematical career, which a lawyer certainly isn't. Maths, government & politics, philosophy & ethics and psychology are a good choice of subjects in my opinion, as three of them are essay based and maths shows that you're diverse and shows your mathematical skills.
I've just started my first year of studying A-levels and I can say from experience that A-levels are a big step up, which I'm sure you've heard plenty, and considering I got A*s and As in everything and took the subjects I got the best grades in for A-level I'd say I'd be a pretty reliable source. So if you're not overly confident with English I'd recommend against taking it. Law doesn't specify that you've taken English, so as long as you've got essay based A-levels you'll be fine.
I take maths, further maths, biology, chemistry and physics so I'm probably not much help at telling you which of the subjects you'd enjoy the most, besides, it's different for everybody. Maths is a big jump up, even considering I got an A* at GCSE; however, I'm still enjoying it loads, and its manageable as long as you organise your time well and commit yourself to it. My friend is taking government & politics and she absolutely loves it, so if it's something you're interested in, I recommend going for it!
Don't worry too much about your A-level courses though. Apply for what you feel is best now, and if during the first few weeks you feel like they're not quite fitting colleges/sixth forms are often fairly willing to let you change as long as its done within the first few weeks.
Good luck for the future, I hope I've been of help!