If it’s just getting used to studying again you might want to try a MOOC course – I personally like Coursera, but it’s by no means the only one out there. Not an A-Level, but they are (generally) free and would certainly help you build study habits in a gentle, risk-free way.
Quick check through their catalog brings up this one: https://www.coursera.org/course/earth – it’s not currently running, but should give you an idea of what’s available.
Not geology, but possibly of interest and starting soon:
https://www.coursera.org/course/oceansolutions
Edit: quick Google, this one is self-paced (which I don’t like, I prefer a deadline to keep me honest) but ideal subject matter: http://www.allversity.org/courses/basic-geology