Any tourist attraction in the UK, especially those aimed at children have to compete hard in terms of entry price. It is not surprising therefore that they try to widen their profit margin in other areas. This invariably means that the catering put on in theme parks, zoos and the like is overpriced, unhealthy rubbish. Of course, many attraction may only take a couple of hours and it is not an issue, but bigger attractions may well be a full 'day out'.
I'm sorry, but five pounds 50p for a medium sized, mediumly warm potato with a few heinz beans on (not forgetting the miniscule side salad) I was recently charged at Longleat Safari park (near Warminster) could never be described as reasonable.
On the other hand they do not object to you bringing a picnic, and provide nice areas to eat it in. Tickets will also allow for exit and re-entry.