My coolbox lasted all weekend (4 days) without EHU when i froze a 2L bottle of water and a small carton of milk. We have a couple of small ice packs too but the milk defrosted gradually and we used it on the last day and the bottle of water still had ice in it on the last day! We drank that on the way home! Don't need fancy ice-packs!
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.