Secondary school mathematics

Madasmaths (available at https://madasmaths.com) - a veritable goldmine of mathematics questions.

Underground Mathematics (https://undergroundmathematics.org) who have a lot of good practice problems for A Level mathematics.

Isaac Physics (https://isaacphysics.org), for both the practice questions as well as their notes (https://isaacphysics.org/concepts?stage=all).

The textbook "Further Pure Mathematics" (and the other books in the series) by Linda Bostock and Sue Chandler. Available to borrow (for free) from the Internet Archive (https://archive.org/details/furtherpuremathe00bost).

Math Centre https://mathcentre.ac.uk (in particular their "teach yourself" section, available at https://mathcentre.ac.uk/types/#h4)

HELM (Helping Engineers Learn Mathematics): features notes and practice questions. Available at https://www.lboro.ac.uk/departments/mlsc/student-resources/helm-workbooks/

Colin Beveridge’s blog, "Flying Colours Maths" which I originally discovered because of a post explaining how to find all the invariant lines under a matrix transformation (https://www.flyingcoloursmaths.co.uk/ask-uncle-colin-invariant-lines/)

Adrian Banner’s \sayThe Calculus Lifesaver (the author can sure draw)

Advanced Problems in Mathematics: Preparing for University https://www.openbookpublishers.com/product/342