Also depending on your destination, consider emergency medical evacuation and read exactly what it covers. Some policies only cover evacuation to the nearest hospital which can adequately treat the condition, others may cover evacuation to the nearest home country hospital (which is not particularly helpful if that is NYC and you live in Los Angeles), and others will get you home. Bear in mind that this is not just for the sick & elderly.

Finally, bear in mind that you will likely need to lay out cash for treatment overseas and then recoup from your insurance. Billing systems and coding don't speak to each other and this can be a big issue. This doesn't mean traveling with massive amounts of cash. But, it does suggest that you have someone back home with your power of attorney to access bank accounts and wire transfer money.

As others note, there are no generic answers to the question. I would not worry about locating a broad spectrum antibiotic in London (and it may be useless for your condition).