I'm planning on doing CS at uni too (I'm the same year as you) and my understanding is that although Oxbridge/imperial can't require you to do further bc not every state school offers it, they expect you to take it if it's offered. taking it may also help get a better STEP grade if you find you need to take that too. I'm taking maths, further, CS and Spanish.
(from what I've seen, most CS students do physics if they don't want to/can't do further maths)