The Trevor Arms is an option if you're going but don't want to eat there. You can book in advance and there's loads of time to drive there and back and eat during the interval. If you're going to Hippolyte et Aricie it's well good. Properly bonkers.
Take your own picnic and don't think the dress code is optional, I've seen a lot of very self conscious men walking around in lounge suits in previous years. Enjoy though, it's fun!