Thank you for starting this thread noblegiraffe
DS got a 6 in his Maths GCSE mock in January; he had been working hard on bringing this up to a 7 or even an 8, doing at least one paper per week.
He very much wants to do Maths A-level and is motivated to continue working, but his school has not (yet) set any work and the long gap will make the jump even greater.
I have ordered the CGP headstart book for him and he has also expressed an interest in trying to teach himself some (AQA) Level 2 Further Maths. Luckily he has an older brother who is good at maths, if he gets stuck.