computing a level has some maths content, most of it is on the gcse course tho and its nothing horrifyingly hard. the rest is how computers work etc. (again similar to gcse), there is stuff about laws and ethics and programming does play a part too. you have to be familiar with programming to answer questions and lots of the a level courses also have a coding project which has a lot of freedom in regards to what you can do, what language you write in, etc.
i do OCR, i'm sure other exam boards are pretty similar.
further maths is good for top unis, i don't necessarily think it'd that hard at AS but it's a lot of maths work so if you're not naturally good at maths/don't enjoy maths then you will find it tedious, though there are people in my class who are doing FM just as a route to computing and they're not coping too badly.
I would say stick with what you're good at. maths is a must, but otherwise just choose what you want to study, you can always do an EPQ in computing to show universities you're good at it/have passion.