Can I ask why you would want to self-teach?
My quick response is: A levels are very challenging; I'm doing 4 and can't comprehend self-teaching, because of the difficulty of content, the tricky mark schemes and exam technique, and mostly self-motivation. Even at school 5 A-levels (only very rarely seen) is a huge investment of time and largely futile, since unis only look for 3 A-levels. It probably is quite unrealistic, then.
You'll probably enjoy more taking only 3/4 so you can gain a greater depth of knowledge and take time to absorb the content instead of frantically scrambling to learn things. Why do you want general knowledge in all of these fields? If general knowledge is your aim, you can read a book, watch a documentary, or listen to lectures; A levels don't give you a broad range of knowledge anyway.