It's not about the quantity but the quality. you can do more in 1h of work if used intelligently than in 5h of semi-work. You're only doing your GCSEs, make sure you do your homework properly, if there's something you don't get, then spend a little more time on it, try to explore new concepts further than if you only looked at the material given by your teacher. Read about what you're interested in, be it Medicine or not, and try to pick up an unusual hobby to keep yourself busy, maybe language learning, an uncommon sport, etc. It will keep you off the Internet, and it may even come up in your interview, I remember a TSR user saying that most of his interview was actually about his reason for doing Russian at A-Level.
Try to explore different ways of learning, find out what type of learner you are (visual, auditory, kinaesthetic) and adapt your learning methods to your profile. Try to make it as fun as possible, and if your friends need help about something you do understand, teaching them will re-inforce your knowledge of the topic. Regarding staying off the Internet, if you're a Mac user, SelfControl is a really good application, unfortunately, I don't know of such a software for Windows/Linux/etc. However, there's LeechBlock that exist if you happen to use Firefox, and it will block the pages you tell it to block (but you can still use other browsers at the time…)
List your goals, and break them down into realistic and easily achievable goals that will be immediately rewarding, because "I want to do Medicine" is too far from you at the moment, whereas "learn the new trigonometry rule" will be far easier to achieve. Make lists and cross the done thing, it always makes you feel good! Some days will be more difficult than others, but don't worry, if you've got a **** day, it can only improve. Good luck!