I'd say do maths as an a level instead of computer science. I do both, but honestly, lowkey regret taking cs. CS will be very time consuming with nea but so can maths with revision. However, Maths can be quite easy to get top marks since you just need practice a lot, and plus its more widely accepted for cs uni.
its ok if u got a 6 in maths, as long as you do work. If you do the questions of each topic/unit u did in class at home, you'll be fine