I believe it would be possible as your only studying one A-Level Subject as i have a relative who managed to do A-level Further Mathematics during his gap year and achieve an A*. If you are willing to create a schedule and give yourself a deadline to complete learning a chapter/Topic by. You can easily achieve a high grade as there is a lot of resources for A-Level Maths and youtubers/website you could utilise to your advantage however you will have to make sure your practicing questions. As there is a lot of content to you need to know.