Bonjour Chloe
I’ll answer your questions from what i remember from working 2-3 weeks in my local mairie, some 3 years ago. The Secrétaire du Maire is an Urban expert there, so she explained a few basic things to me at the time.
First of all, go to your mairie / phone them to make and appointment with “le / la responsable du PLU”. Ask for that person to explain the decision in detail : dates of PLU, the map of what is constructable , etc. and why & why not. Take notes in front of that person and get his/her name. Just to get the right info and that you understand what the score is. Nothing to do with Brexit, no. Anyway, it wouldn’t make sense.
You ask : Why on earth would the designation be different now when nothing has been changed since the last certificate was granted?
Is this possibly a response to Brexit? Is it possible that building plans are under more scrutiny than before if they are submitted by an English resident?
→ The Plan Local d’Urbanisme must have changed. Ask them about it. It is important. If nothing has changed about the Code de l’Urbanisme (that you can find on the net, btw), then you shoud have 18 month after that date when your land is still constructible, and to sell it this way too, etc… It’s a bit technical but not unsurpassable 
Decided by the Maire & conseillers, the PLU can change, say, every 5 years or so if needs be. You will not get personal notification but when a change is made, it is put up on posters outside and sometime inside the mairie too, by law, for a certain amount of time. You might even find the notification on your Mairie’ s website.
In our village, the new PLU was challenge in court by someone who wanted to sell his land by developers. Year after year, it was denied because of the PLU stating that hhis land was not constructible. Well, he won his case because the reason for the new PLU was not “substancially explained” - just a wishy-washy explanation instead, about preserving the beauty of the area.
So our PLU had to be binned and the old one put back in place (2011), so nothing new for the poor guy, anyway.
yOU ALSO ASK : If the land is deemed “un-buildable” for habitation, do we have any options to sell it? We paid 2,000 to have the land transferred to our names based on the fact that it was worth 30,000 - now it is barely worth the price we paid.
→ Who sold you the land ? A notaire ? Private seller? Estate agent? the SAFER?
Challenge them on what they based their price on. If he/ she is a professional, they can do so and should do so ! If they can’t, they can be in trouble. Your choice.
Also, sorry to say, but “you should have made your own homework” and go to the Mairie to check everything about he land you were buying. If it’s agricultural or humid or preserved land (acc. to the PLU) you cannot build on it. If their is at least ONE old wall on it that furthermore appears on the CADASTRE, DO NOT DESTROY IT. It means that you can potential build more - or even extent a house / renovate a ruin + building an extension.
Contact the SAFER, specialised in selling properties in the countryside : http://www.safer.fr/
Also, if needs be, get help from a friend / a neighbour/ a professional “facilitateur” ( French/English “helper”) on the matter.
If you have a Protection Judiciaire - sometimes attached to your Assurance Habitation - you need to contact them for advice. Sometimes they can also assist with taking someone to court (paying part of an “avocat” fees). Sometimes the PJ is expensive but well worth it. I’m with Pacifica (from Crédit Agricole bank). They have been really good over the years.
Good luck with everything - I’m sure everything will be ok in the end, hang on there.
France is a damn challenge (I’m French :D) and if you are here, it was your choice 
Let us know how it goes.
Christine Borrow, from Sunny – now anyway ! – Vendée
Do we have any way of questioning the decision to withdraw the building designation? I am in the process of translating the various reasons given for denying the certification, but if we are able to fix the items they deem to be problems, can we simply re-submit?
If the land is deemed un-buildable for habitation, do we have any options to sell it? We paid 2,000 to have the land transferred to our names based on the fact that it was worth 30,000 - now it is barely worth the price we paid.
The biggest problem for us is that we had been relying on the sale of this plot to help finance buying a house for ourselves: we are currently renting my mother-in-law’s house which is also on the market, so there is a slight urgency to all of this.