son and his friends went last year - they flew as we live in Scotland
if rain is forecast then make sure they know how to pitch their tent correctly and keep everything in poly bags within their main bag as it's miserable with wet clothes and nowhere to dry them. Try to persuade them to take one jumper/joggers/socks more than they think they need as they will be grateful to you in the end. Thick bin bag for their sleeping bag during the day if it's raining. Keep one set of clothes for travelling home as everything else, including your child, will be rather fragrant by then!
camping chairs are highly sought after and make sitting around their camp much more comfy - just don't leave them out and don't expect them to come back.
mobile signal is terrible although fine if they go into town during the day so expect day time messages from them. The main issue is if they get separated from their group/plan to meet others within the area at night then they simply cannot contact each other and the main areas are too busy at night to chance on finding people. Best plan is to always be with at least one other person
the food options are fine but expensive. The Aldi in town was visited daily and there was a cafe in a converted church that they went to daily for decent food and toilets
there is a safeguarding team - if they are under 18 and trying to bring in alcohol then they will probably be caught and you may get a phone call from the safeguarding team (several of us parents got said call over the 5 days). The most successful method was to buy a box of wine from Aldi and tape the bag of wine to their backs and wear several layers of clothes when re-entering the grounds! Drugs were available but easily avoided
there are several different camping areas - our lot had decided in advance where they wanted to be and set up camp together on arrival. The final night was when there was trouble - groups seems to go round wrecking tents and stealing. My son and his friends are big rugby lads and seemed to take it in turns to stay by their tents/stay awake to defend their area which meant that the girls felt safer than had they been alone. There was nothing significant stolen but they definitely need to keep their valuables on them at all times. Have low expectations of tents/kit coming home again. If there's an option to leave late Sunday night then take it!
that said, they had a fantastic time!!