The current 'roadmap' is for large outdoor events in May and all restrictions lifted in mid-late June. Some outdoor festivals scheduled to take place in May would could theoretically announce they were going ahead.

However, you'd have to hedge your bets as a promoter that every target date gets met.

Having been one of the most conservative, following-the-rules-and-then-some people I know during the entirety of the pandemic, I am reasonably hopeful about the situation from late June. Personally I would attend outdoor events from May and I am optimistic enough about the post-vaccine R rate effects to anticipate that indoor events would be safe enough for me from July onwards.

Just keep checking individual festival pages.

No way the government will introduce a policy vaccine passports for entertainment events; that ain't gonna happen. But I think legally a festival might be able to introduce its own policy; some businesses are already talking about doing this.