Edexcel’s transition to A Level Maths videos and worked examples are a very comprehensive set of free resources designed to help the transition from GCSE to A level Maths. The content focus is on the overlap between GCSE Maths and the first year of the A Level Maths course making the resources useful not only for initial transition but also for the first year of the course.
From Sparx Maths, have a look at their Transition Booklets, one for Year 6 to 7 and one for GCSE to A Level; these booklets are free whether you have Sparx Maths or not. The GCSE to A Level booklet uses over 120 questions taken from 11 key topics on Sparx Maths. Answers can be obtained by completing a form with your school details.

For students going on to A Level then a really useful publication is use OCR’s brilliant guide for students Bridging the gap between GCSE and AS/A Level Mathematics – A Student Guide. With sections on Algebra, Trigonometry and graphs including examples, question practice on key topics and suggested reading before starting the A Level this is so valuable for students.

Free lessons from Colin Hegarty recorded on YouTube are available to help GCSE students prepare for A Level Maths.
From TL Maths GCSE to A-Level Maths Bridging the Gap is a series of videos looking at some of the basics of GCSE Maths Higher Tier that students should be proficient in before they start A-Level Maths.
You could also look at Step Up to A Level Maths from The Centre of Innovation in Mathematics Teaching which helpfully lists skills you should be confident with and provides resources to support your study of these skills.

From AMSP – check Transition to A Level Mathematics – Essential Skills.
These Transition Takeaways from Mohammed Ladak have been specifically chosen to help with A Level Maths preparation.