I personally didn't use flashcards but if you found flashcards to help you well in GCSEs then I don't see why it won't help in your A levels.
For Biology, I wrote out my notes and used a blurting technique so I would write out everything about a topic closed-book and later compare it with my original notes which is good for active recall. Flashcards would be good for this as there's a lot of memorisation involved.
For Chemistry I just wrote notes and did exam questions from physicsandmathstutor after finishing a topic. I guess you could make flashcards for the organic chemistry and synthetic routes but there isn't as much memorisation as there is in Biology.
Lastly, the best revision you can do is taking past papers in timed conditions and getting a teacher to mark it. This is the best practise, especially with Chemistry having a lot of application and maths so its best to get familiar with the questions early.