Hay Festival (is it Hay or How the Light Gets in you're going to?) has a huge food tent with loads of providers offering food from around the world. It's not cheap, but at least you're not trying to slice a crusty sourdough loaf with a blunt knife supplied by the Air BnB. Hay has loads of pubs, restaurants and food outlets. There's a food court in the castle entrance area most years.
They'll be crazy busy if you're there over the weekend but during the week it'll be quieter. There was a very good deli in the main street last year (presume it's still trading). Really excellent pub (Three Horseshoes at Groessford) if you have a car/ fancy a short drive to get away from the crowds. Not cheap but very very good. We've had good food at the The Three Tuns in Hay itself for the last couple of years and would recommend. It's slightly out of the way and tends to be ever so slightly quieter than some of the others. Decent fish and chips from the chip shop near the Three Tuns.
My menu would be bircher muesli with yoghurt and berries (strawberry season!) or a bacon butty made with good bacon.
Lunch, if I was taking something to eat at the festival, would be pasta salad with hot smoked salmon (pasta, creme-fraiche/mayo, peas, seasoning) on a bed of watercress with chunks of salmon through it. You can substitute the pasta with Jersey Royals.
Day two I might make a good salad and add a slice of quiche or a pasty from the deli. Buy an almond croissant or chunk of cake at one of the places in the festival offering cake and coffee mid-afternoon.
Dinner: if I had evening tickets for a festival event I'd probably eat in the food tent. If not, and if I needed some peace and quiet, I might head out of town to Glasbury or Groessford to escape the madness. Or buy fish and chips and take them down to the riverside to eat them on one of the benches: take the steps down by the bridge to Hereford and you'll find a lovely riverside walkway which is shaded in the heat. You can walk along there to the town 'beach' at the Warren near the festival end of town, though last year the Wye was so polluted that no one was going in the water...
(Yes, I've been going to Hay for years and hope to be there next week)