If you're going to go into biochemistry, you should take... biology or chemistry. I'm in biopsychology, and, of the pure sciences, I do find biology the easiest. People probably say its the easiest because it involves the least math.
But I think the most important thing to question is... if you are avoiding biology (by leaning towards physics or chemistry), why are you wanting to go into biochemistry?
As to what math is required, I sugest contacting your advisor. Wait, don't just contact your advisor, become close friends with your advisor. Your advisor should know exactly what career goals you want. What graduate school you want to go to (you have an idea, don't you?). And what specific field you want to go into (you have an idea, don't you?). And where you want to work (you have an idea don't you?). These are important and though you think you're smart (and I think you're smart, too), your advisor has more life experience in his/her pinkie toe than both of us have combined. It is available, so use it.
Unless someone here happens to be a science college advisor in England, we're not going to have the experience and advice you need.
Things to Bring to your Advisor.
Major: Biochemistry
What you want to study with this major: The brain? Animals? Are you going to be a research scientist or an applied scientist?
Where you want to go to graduate school: Look up some schools with a major that will get you closer to the above major. Bring it to your advisor and he'll help you narrow those schools down. Use your current knowledge in what kind of colleges you like. Big school, small school, near a town, abroad, ect. You and your advisor will turn it from a pool of ideas to a prioritized list.
Where do you want to work: If you're going to research, do you want to research and teach in a college, or the government, or with a private institution? I've never had a desire to do applied science, so I don't know much about where you'd work in that, but... look that up! It's important! Because I want to work in the government... I'm learning chinese to improve my chances of getting hired!
---
When you WRITE IT DOWN, THAT'S when you'll realise, "Hey, this is the dumbest future I could have thought for myself and I would hate living this life." And then you crumple up the paper and just write something else down, no biggie. Much easier than changing middle of your junior year.

So write it down and take it to your advisor!
