Honestly I can't belive what I read on here sometimes.
You can't believe it, or you don't want to believe it, or you don't understand it?
Who ran the abbotoirs pre 2000s?
Utterly irrelevant - it is now 2019 and the world has moved on. If the European vets move back to the EU, accepting the Brexiters' invitation to bugger off back where they came from, then 90% of all meat production immediately stops in the UK.
Sorry how much food do we buy from the the eu? It's about 30 %.
30% of food is produced in the EU. Much of the rest of the imported food, e.g. from Africa or South America, comes to the UK via Rotterdam and Hamburg and so will be delayed or in short supply along with the EU food.
I've posted above about the importance of ingredients - the problem with food is not just limited to finished goods. A typical 'British' pizza uses cheese from Ireland, tomato paste from Italy, herbs from southern Europe, mushrooms and peppers from Holland, ham and meat from Denmark and so on. A delay in any one of these ingredients stops the whole production line.