In reply to freeheel47:
Intensive care doctor here. Hence this is not my area of expertise. The following are some observations though:
1. Depends on where you're going and how far you're likely to be from medical support and what the potential patient is like in terms of any pre-existing conditions
2. In a remote setting would happily give abx in situations I judged serious. I would not remotely give them for your average tummy bug/cold etc so that someone could get over it one day earlier etc and resume the expedition.
3. Clearly the safest way is to get them from a doctor you've spoken to and knows your history and the likely possible pathogens etc etc. Doctors are criticised all the time for over-prescribing abx and there is a looming resistance crisis pretty much worldwide. Hence many doctors will be reluctant to prescribe abx for some undefined possible future complaint to someone who is well.
4. Good question. There are relatively few infectious problems that are so acute and so serious that you couldn't get to medical help in country in most places. Admittedly it might inconvenience your expedition somewhat.
5. Depends on your GP's approach I suppose. I think many would see this as not exactly within the usual scope of NHS work.
6. Most doctors are used to this from friends etc but won't commit very much without (a) seeing the patient in an appropriate professional context and (b) it being within their area of expertise.
As a sidenote I would be very very cautious about just buying stuff and hoping for the best. Abx are one of the commonest classes of drugs that cause anaphylactic shock: this is a life threatening emergency even when it occurs in hospital, hours/days from high quality medical care it could have catastrophic consequences.