I don't have much experience in A-level biology but in regards to chemistry:
In my opinion can actually get away with using a combination of exam papers from the old syllabus, youtube AQA chemistry channels and buying the AQA book which although expensive is till cheaper than £50 / per subject.
You just have to make sure to download the specification of each exam (free download) and check which topics are in AS level, check old exam papers against the specifications and solve the relevant questions. It can be a great research project for your child to identify current AS level topic questions from old exam papers and the next week they can actually solve them.
The only thing that cannot be found in old exam papers is the multiple choice questions, but you can find examples of those at other exam boards. Again, just make sure you get the topics right as other exam boards might distribute topics differently.