1. As much as 40-60% of medication sold in the third world is counterfeit, especially the modern, expensive stuff.
2. You can take 2 months supply with you from your NHS doctor; get a repeat prescription and I think you can get more than filled and shipped to you.
If you need to source overseas, use a reputable private clinic that imports their own supplies. International SOS are expensive, but reliable. If you plan your stops around first world countries such as Singapore, Australia, South Africa, Australia and New Zealand, Canada and America, you will not have a problem but may have to see a doctor privately to get the medicine prescribed which adds to the cost.