Each festival will have its policy; it is worth checking in with them. Glastonbury, for example, announced it would roll over the tickets purchased for 2020 for entry to Glastonbury 2021.
If you bought your ticket from an official seller, then you should be entitled to a refund if the event is cancelled or postponed. However, this may not be the case if you bought from a re-selling site or private seller, this may not be possible.
Find more info here: https://www.citizensadvice.org.uk/consumer/event-tickets/event-tickets/getting-a-refund-on-an-event-ticket/