Depends what type of scientist you want to become. The minimum is good GCSEs and A levels including maths and at least a couple of sciences. For a job in say an industrial or hospital lab doing non-research based science then a degree in a science subject is usually required. To be a research scientist then a higher degree, usually a PhD, is expected which now takes an average of 4 years to finish and involves doing an extended research project.

That depends entirely on what sort of science you want to do! Being a doctor is a scientist and that takes lots of exams! Mostly, you just need to do science A-levels and pick a degree in one of the many, many science subjects out there. Science is a huge subject and there are opportunities everywhere.

Nowadays, anyone can become a scientist in the sense of participating in research projects. This option is called Citizen Science, and it covers ever more fields of science. Just a few examples: Fold it is a 3D puzzle that helps determine the structure of proteins, and the Open Dinosaur Project collects measurements of dinosaur bones, and Galaxy Zoo helps astronomers structure the knowledge about galaxies. You can find a whole range of such initiatives at Science for Citizens, and I keep up with citizen science-related news via a dedicated room at Friendfeed.

If you engage with one or more of such projects early on, you will quickly gather useful experiences and find out about the range of formal qualifications required and/ or useful for the professional pursuit of science in the area of your choice, be this in the UK or elsewhere.

Another option in this direction is to directly contact (perhaps with the help of you science teacher) some scientist, either online or at a university nearby. Most of them will be open to inquiries by motivated students, and virtually every scientist working in publicly funded institutions now has a public website.