So you're home educated.
If you do poorly on your first paper of a subject, but very well on the second, you can probably still scrape a pass yes. A grade 4 perhaps. If your first paper is absolutely atrocious then expecting a 5 or 6 overall is a bit ambitious, but a 4 is a possibility.
Bear in mind that CGP books don't always cover absolutely everything. There were some things on my daughter's Biology exam a couple of years ago that weren't in the CGP book.