We usually take our own but go back to the car to eat. The food is quite expensive and you will be queuing for ages if you eat at lunch time. Best to eat early or late if buying.
Yes you can take in your own food & drinks & I would advise you to do that because the price of food in the show is very expensive & I mean VERY expensive.
I don't think it's just your horse live, I have been shocked at the prices hot food vans are charging, I pity people feeding families, makes for an expensive day out