I wouldn't risk such a close timeline. If the coach from fest takes 1 hour and a taxi from coach to airport takes 30 mins you're only there 2 hours before your flight and this doesn't consider traffic issues getting away from the festival, traffic issues in Bristol or any other extenuating circumstances.
At Glastonbury no chance. Cave would be really selling himself short as well if he took the Acoustic slot.
It didn't engage the masses with the bad seeds last time to be fair. Even for a band after that were absolutely massive at the time it didn't get the usual we'll sit through this for it.