The school may not have a Brexit plan but they will have contingency plans for most of the situations that people are concerned that could arise.
We were a military family with dc at boarding school. Based at 1 end of the UK from their school at one point. A very long drive but I did it in a day once in an emergency.
We were also abroad at one point.
We lived through the Ash Cloud delays/flight cancellations, Foot and Mouth disruption and road closures and the petrol crisis - even with our military allowance we could not get to school, it was so we could get to work.
My kids survived and even enjoyed their extended term and the adventure it threw up.
From memory due to the petrol crisis and their rural school being in a foot and mouth area (2 different occasions between 3 kids) the school did run low on supplies and have stranded students. They managed for the time. A few odd meals but they loved it. Some local families took some kids to stay, some stayed at school with also stranded staff. School went on with extra fun activities etc.
The Ash Cloud was chaotic. My dc were ferried back and forth to an airport several times. School were fab. Had staff on hand, remained open. 1 dc ended up staying at a friend house for part of it. The Ash Cloud crisis came out of nowhere. I don't think there was a boarding School without pupils affected. Many boarding schools just had to deal with the fall out there and then as it unfolded. No one I know- and I know quite a few military families with dc or had dc at boarding school at that time complained at how their school reacted. Not the most pleasant time, but well dealt with.
I'm sure if there are kids trapped in schools with no water, food and heating they will be higher up the list than those kids in their own homes with families that also didn't prepare, to get help. The military will be sent in to do something like they always are at a time of crisis.
Personally if I was as worried as you I'd be sending supplies and picking my daughter up well in advance of the Brexit apocalypse to cover both bases.
Have you thought about contacting other parents at the school to gauge their concern? Do you have emails or a parent contact list? Social media page you can tweet or post on? You may find others may have better ideas or want to lobby your school to get a Brexit plan asap with you!
Foot and Mouth was before internet was big for us. No online orders etc. But today you could easily start your own planning. Get onto Amazon Pantry and send your dc bottled water, tinned food, packet foot, multi vitamins, water purification tablets and even ration packs and a little stove.