Firstly, any decent computing uni will require an A at Alevel maths as an entry requirement. Secondly, IMO it's better not to have had any computing experience before uni as it can get you into bad habits that are hard to shake. I think that a scientific, technical mind will be much more benificial than a BTEC. On top of that, you really do need the maths, without it it's a very, very steep climb if you manage to get into uni.
And just to put this in context, I did a BTEC, made the mistake and learned from it

I applied for Comp Sci @ Southampton and I know it's really not easy.
My advice remains, stick to maths and sciences.
Burnsy