Homemade Yoghurt - boxed UHT milk or a good quality powdered? Maybe a mix of both?

Having lived in a country that also where dairy was not widely or easily available, I can empathize. UHT milk can be used to make both yogurt and ricotta, as can powdered milk. UHT milk can't be used to make cheeses that depend on rennet, though. I used to order whole milk powder via Amazon to mix with the ordinary powdered milk because it is usually skim milk. That's low in fat, and increases shelf life, but a little fat in yogurt makes it a bit tastier. You can also purchase dried cream powder through Amazon as well, which is handy for making marscapone (should you want it), cream cheese or even whipped cream. You can buy dry cultures to make many of these things on the internet as well - my favorite source is New England Cheesemaking Supply, but there are others. That site will walk you through the process and provide recipes as well. That can come in real handy, and the folks there are great to answer questions and help you succeed.

You can use UHT milk, should be ok. Usually you add milk powder for a thicker yogurt regardless of the milk used. So It might be a good experiment to see what is the perfect balance for you.