The only ones I’ve done were in Knock Off, at the Butlins Punk Weekenders. 4 bands play on the introducing stage each day (3 days), the audience vote for their fave of the day. Fridays winners then play the main stage next year on Friday etc. But to get through to that point required (requires?) applications as only 12 for a whole weekend. And although they’re introducing stages they are still big, with good backline and good audiences as the main stages only open at night.
@SteveXFR, might be worth checking the Minehead one out?