You don't need any previous computer science to study it for a degree only maths and further maths.
Ds1 is in year 13 (when they go back) admittedly he has done computer science for both GCSE and A level, his sixth form just want you to have a level 4 at GCSE if you took it.
Even Cambridge are only interested in your maths skills. They don't want computer science A level. Maths and Further Maths at A level are needed for the top 6 universities for Computer Science, so Cambridge, Durham, Warwick, Manchester, and some London ones.
There are a lot of online things to look into without taking a GCSE in it. Ds1 and Ds2 for that matter have always done online stuff for fun outside of school computer science. If they are interested, they will find a way.
Obviously it is shit that they have cancelled the course but better that than a shit teacher. The spec for the course is online so your DD can look at that to see what they cover in GCSE. But this is about maths, maths and more maths.
If this is the route your DD is looking to go down, then you need to find a sixth form that does FM for the future. Yes, you can apply with just maths but a candidate is severely disadvantaged if they don't have FM too.