3u maths was my worst subject, my raw assessment mark that i ended up handing in was 86 and i thought it was literally impossible for me to get a band 6 (E4 for me) especially after i approximated an 80% raw mark on the actual paper.. in the end it came out with a 47 i.e 94.. dont doubt that you can get a band 6.. it is possible! especially if you are doing 4u then you are better than me and can definitely do it...
In terms of what marks you need to get, considering u have reasonably similar subjects to me, i've been playing around with uai calculators etc and it would really help if you did well in the sciences they scale very well if you go well in them, obviously you need a good english mark but ive found its helpful to get a high advanced mark as the extensions don't do as well as you would expect. Overall, you can afford to get some a high band 5s... but if your english, phys and chem are in the mid band 6's you'll do well.
on another note, i notice you're uni aspirations are exactly the same as mine, i want to continue physics and do law.. are you planning on doing combined science/law?