I did it this year. I got my 3 A Levels in 2015 but I needed one for University for my course. Therefore I decided to take a gap year and teach myself the full AS and A2 course. As I worked for half of my gap year, I only truly spent around 4-5 months studying and revising for the exams. I finished them 3 weeks ago and obviously im waiting for results. It felt good to be able to study at my own pace, there was difficulty with not having a teacher or private tutor, especially for my speaking exams and essays, but luckily TSR was on my side and I got all the help I needed.
It really depends on whether you feel confident and independent enough. I'm definitely not going to Oxbridge, only to Birmingham, but pending results, I think this was a good thing to do for myself, it's definitely doable. You just need the time, capability of sticking to the course, and a positive mindset.
Good luck with it all!