Hiya so if you're not sure exactly what part of Science you want to go into, definitely consider the Fast Stream! We have a specialist 'Science and Engineering' track that's designed precisely for people who love science and also want to work across a wide range of areas.
The entry requirements are quite broad and for this particular stream. You can apply with a 2:1 in any undergraduate degree subject, but on top of that you do need to hold a doctorate or masters in a biological, physical, mathematical science or engineering subject (or be a registered engineer or scientist already!)
If you decide that going into pure science isn't for you, there's still a chance to work on science-related things as a Generalist Fast Streamer. I've got friends who have been working on the UK's space research policy, on funding for science projects, and on agricultural science, even without having studied a single science subject at A Level!
So basically, if you are dead set on science then taking science subjects may make the most sense, but there are options out there that don't require you to study science at University if you don't want to!