I am not now nor ever been in the SAS, SBS, etc. I will be starting in sandhurst sometime in the next year-ish.

You have two options of entry. Soldier or Officer. This is what you should decide first.

When you're a doctor you can commission straight to the rank of Captain and you'll be assigned to the Royal Army Medical Corps. http://www.army.mod.uk/army-medical-services/5319.aspx you will have a shorter stint in Sandhurst but you will not be part of the SAS (there may be some routes around this but not sure and doubt it). You can chose not to go to the Royal Army Medical Corps but then you give up the captaincy and just follow the route of every other officer.

My best advice is to start the officer application immediately if interested. Apply for a form on the army website. It takes a long time. (From my first contact to when I start is about 1-2 years.)

If you want SAS(R) though that's the TA. You should join the TA! Get thee to your local TA recruiting office! Another option is to join the OTC in whatever university you'll be attending or one that's nearest to you. This will give you some sharp and severe lessons in personal admin and fitness. Everyone loves the romantic idea of being in the SAS but the horrible reality is that it is probably not as sexy as you imagine.

Resources http://www.army.mod.uk/join/join.aspx - obvious one! http://www.arrse.co.uk - british army forums, lots of info, wikis, forums, humour, etc. protip: don't post here without searching thoroughly or you'll be ripped apart but in a loving and tender way so that you learn from your mistake.