Language is the most important key to law. That said, you need to be able to reason well too, so logic abilities are important. In my opinion however there isn't that much I can see a mathematical background would bring to the learning process in law. Philosophy courses in logic will equip you to deal with reasoning and learning in law far better than any amount of maths or science. Of course this doesn't mean you're at any disadvantage for having these skills! You should be happy you have a broad spectrum of abilities.
Surely though if you're doing 4U English you can't be that bad at English? Maybe you're being too hard on yourself. Capacity is different to motivation obviously. If you can do well at English but you don't like it, then so long as you work, you'll be fine - it just means you might not enjoy the work as much.
Contrary to popular belief there isn't an excessive amount of reading in law. Sure, there is probably more than other subjects, but it is often more the demanding nature of the reading than the quantity.
If you can tackle 4U English and 4U maths I'm sure you can 'succeed in this field'. From what you've said seems to be more about would you want to be in this field?
Oh, and nitpicking is good
