The legal procedure is outlined in my post from yesterday evening, here:
https://www.theanswerbank.co.uk/Law/Question1650890.html
Drivers 'borrowing' cars is far from unusual. Twenty years ago I worked as a trade plater, delivering cars across the country (mainly to auction sites). The firm I contracted for employed hundreds of drivers and, to the best of my knowledge, every single driver (including me) falsified the mileage of the last vehicle they picked up on a Friday afternoon (which was meant to stay outside the driver's house at the weekend and then be delivered on Monday morning), so that they could use it themselves over the weekend. (Where possible we also deliberately over-fuelled those Friday afternoon cars using the company's card, so that we'd got 'free' fuel for our personal use).
I never 'borrowed' a Friday afternoon car for more than a couple of hundred miles but one driver got caught out when he pranged one in France. Another driver let his mate borrow one, who then wrote it off in Scotland (when it should have been in Suffolk).
Local garages frequently 'borrow' customers' cars for their own use. I took my car into a garage in Norwich just as they opened on a Saturday morning and asked if they could do a little job for me before they closed at lunchtime. They said that they couldn't guarantee it but I should leave my car with them and call back half an hour before they closed. When I did so they apologised and said that they couldn't get around to looking at my car. They changed their mind, and did the job for me, when I told them that I'd been sitting in the cafe over the road all morning and I'd watched them use my car four times that morning as a courtesy car to run other customers home ;-)
NEVER trust anyone else with your car keys!