The Salzburg Festival Don Carlo will be live-streamed by Medici on Friday 16 August at 17:30 (4.30pm UK time). Good job I'll be seeing in live, as the stream isn't available in Austria. I am told that if you can't see a rights message on the page now, then you should be able to view it on the day in your country. It will also be shown on Arte and ORF2, which are unlikely to be available in the UK (but give it a try).
It will also be beamed to a few cinemas. None in London that I can see, but if you live in Ambleside, Chichester or Ludlow, Friday is your lucky day.
*UPDATE* The Arte broadcast will be available in France and Germany only, and ORF in Austria only (plus satellite). Medici will stream live everywhere except these countries, and will be available on demand 'later'.