In the past, Emily's said they do have contingency plans for unlikely, high-risk events like a global pandemic, nuclear war, etc.
The bad news is that both GFL and the UK government will be actively considering whether to cancel large public events (of which festivals are just one), and there will definitely be scenarios that exist in which Glastonbury is cancelled, whether voluntarily or not.
The good news is that there would be plenty that would be cancelled ahead of Glastonbury, so we'd have a pretty good early warning system, and I don't think the current situation makes it likely that this would happen.
Also, as I understand it, in the areas of the world which are being pro-active in containment measures, numbers are being managed or even coming down. The big trouble spots are the areas in the world with dense populations that have large numbers on the move in places with minimal to no containment and limited resources to cope. In other words, once again it's the poor that are fucked.
From a selfish perspective, my flight in June goes through Hong Kong. I'm only transiting, but I know an enforced quarantine of even transiting passengers in hot-spot areas would happen a lot earlier than Glastonbury being cancelled.