I had a brief discussion on the subject when I was reinsuring four of my old wrecks last week. The insurance co rep really had nothing much to say on the subject other than if it's not required they (that's her co) are not insisting on it. Three of them were retaxed this morning. I had to tick the boxes saying they were over 40yrs old so no MOT required and that was it.
I imagine a lack of annual mechanical oversight is likely to lead more to breakdown than accident, but that won't bother the government if nobody else is involved.