Oh it is probably the same here. I don't think i have ever officially written one off.
Just the expression made me think of 'off the tarmac' which is where i personally collect dead cars. A few are also what are called 'SORN' which means it is not road worthy and without tax and MOT but can be kept on private land, which is where mine are.
Yes the more i look at it, normal people do 'write off' their cars. I love the English language.