How do I learn maths?

I forgot maths. I did maths for my GCSEs and A-levels, but I did bad and I want to learn maths.

Is there any book that will teach me maths? I keep coming across books by K.A. Stroud. Are they good? Should I get those?

Keep in mind this is British maths, not American math, so if there are any differences, I would prefer the British version.

Keep in mind this is British maths, not American math, so if there are any differences
So the notion of PI is different depending on the country you live in? And they told me math is universal! Fucking Jews!!!