For GCSE, I love Corbett Maths - the worksheets are excellent and find their way into a lot of my lesson plans.
I have a full set of recent Pearson Edexcel books for GCSE and A-level (and quite a few others from other publishers and exam boards). I use these for exercises for lesson plans, and to see things from the student perspective. I also have a big collection of general mathematics books and bookmarks which aren't geared toward particular exams, which I use both for creating lesson plans and to keep enriching my own understanding.
For assessing where a student is, I make my own RAG (Red-Amber-Green) checks, which I use as a tool to discuss with the student how maths has been going over the past year or so. For GCSE students, I also find that the Corbett Maths 'Five-a-days' can be great to follow that up with.
I have built up a collection my own lesson plans, which incorporate examples and exercises from all of the above. I'm able to re-use them to a degree, but they also tend to become somewhat bespoke. This is because two students' grasp of a prerequisite topic may be very different from one another.
My sessions are very interactive and collaborative. This means I needed a technology setup at home that lets both me and the student write on the same digital 'whiteboard'. I tried a few different things before arriving at my current setup. It works pretty well, in case you're a tutor looking to do the same kind of thing.
Here it is:
I like it because:
The student and I can write on the same worksheet PDF (unlike say Excalidraw, which is excellent for what it does and which I sometimes use as a backup)
The 'Starter' subscription level lets students access the whiteboard without having to log in to Miro. This makes life a lot easier than other products (e.g. Ziteboard, as far as I could tell) as the student can access the whiteboard via a link instead of having to create a Miro account.
I can make as many whiteboards as I want. You do get three on the free version, but more is definitely better for me.
I tend to prefer Linux to Windows or Mac, and Miro runs in a web browser - unlike, e.g., Microsoft Whiteboard, which you have to install, but which isn't available for Linux (and which didn't let me add a PDFs to a whiteboard, for that matter).
We have a Microsoft 365 'Family' license at home (as it doesn't cost much more than the 'Individual' license), which for me is good because the Teams sessions can now be as long as I need. I usually finish sessions within an hour, but not always, and on the free version it cuts you off at 60 minutes.
Cheap, reliable, and works fine for what I'm doing. If you're one of my students and can't buy a graphics tablet at the moment, I'm happy to provide one. Using a graphics tablet makes a definite improvement over trying to write complex equations or graphs with a laptop trackpad or a mobile phone touchscreen (but a mouse also offers some improvement). Of course, if you have an iPad or another good tablet, that may work even better for you!
Online conversations are easier when wearing a headset because the sound is so much better - so I bought a bluetooth one, but it would sometimes cut out at inopportune times. I got this basic Logitech one instead because it's USB, making it very reliable, and the sound is fine for tutoring.