Hi Scottishnerd!
Yes, teaching yourself economics shows you have a real interest in the subject, and have taken the time to improve your knowledge. Intellectual curiosity and "self-directed study" are the top qualities Oxford is looking for! So yes, do mention it on your personal statement. Maybe talk about what you found fascinating about it, any surprising and particularly interesting things you discovered whilst you were teaching yourself? Don't just say "I taught myself A level economics! Oxford really like to ask the question "why". This happens both in interviews and tutorials, so give them lots of "why" answers.
Also Mona mentioned looking at newspapers and journals, so keep on making your knowledge current. There must be hundreds of articles about how coronavirus is affecting the economy, both here in the UK and all over the world. And hundreds of predictions about what will happen next.
I once helped a friend get in for Cambridge economics (she is now in her second year). Her favourite subject at school was maths, which is why she preferred the Cambridge course. If you look at the Cambridge reading list, there is a book by Varian. This book was recommended to her by a tutor at the Cambridge open day. She bought and read the book and yes, it did come in very handy. To answer your question: yes, the Cambridge economics course is very mathsy!
@Mona123456, a lot of students will be choosing between the Cambridge and Oxford economics degrees. I know you have not started your course yet, but is that why you applied to Oxford? Was it the management element you liked? Or is there still quite a bit of maths involved?
In fact, maybe when you start the course, Mona, you could give us an update on exactly how mathsy it is!
Thank you.