Brexit fallout

Over the last ten years or so I have managed to accumulate five classic cars in various states of disrepair. One of them is completely restored and runs perfectly and I use it regularly when the sun shines. The other four are in various bits & pieces, one is stored in a shed in work, two are in my mothers garage, and I have one at home (plus the runner). The one that is restored is on ZV plates, all the others are still on UK plates. I’m retiring in 18 months’ time and my intention was always to restore them one at a time when I would then have the time on my hands. I know the best laid plans can go down the toilet and all that, and it’s likely there’s two I’ll never get to, so I could sell them when the time comes, but the others I want to do. I knew I wouldn’t be able to restore them until I retired , but they all came up at the right money, so I pounced on them at the time.
The problem is that I was hoping to restore them and then register them here, using the , eh. , usual , eh., up-to-date paperwork. I can’t trailer them to the VRT place now as they are non-runners and can’t be registered unless they’re running as far as I understand it, so my question is, what will the position be when Brexit hits, in whatever form and whenever?? If I restore one and present it, will there be VAT & import duty to pay? If I can prove I brought one in from the UK say, five years ago, will they use the old system but charge me the back-dated VRT but no VAT? Are there any official guidelines for my situation? There must be plenty more like me out there with motors in many pieces on the garage floor and still on UK & NI plates.