DS has processing difficulties and HFA. He got Bs in all the subjects he struggled with (sciences) which he was really pleased with and so was I (predicted Cs.)
What helped was:
Finding out how he learns. He tracked down a lot of science You Tube videos and stuck with the ones that actually helped explain stuff.
He did a lot of practise papers with the answers at his side. Almost reverse practise. See what the mark scheme is and then look at the question, so you see what sort of answers they are looking for.
A LOT of what helped him was exam technique.
Another thing that helped was making him explain stuff - as though he was the one teaching it. That was really difficult as I have zero knowledge of science - only took one and didn't do well at it, thirty years ago. So he had to explain to DS1 who is a science geek and very confident and quick to jump in to explain what DS2 was doing wrong. But it did help.
Bitesize and GCSE pass notes help a bit but they seem aimed at C grade passes, not A stars.
For English Language, nothing beats reading. Reading anything - a non-fiction book or a newspaper, so long as its well written. If they do that regularly, they find it easier to write fluent sentences and paragraphs and have the vocabulary to do so.