I’m sure I remember reading somewhere that legally you don’t actually need ‘car insurance’, but you do need some sort of bond to cover the costs of a claim. Consequently, car insurance provides that means. You could therefore give a third party a bag of cash and insure yourself. Apparently, this is what the celebs do.
Yes, to satisfy third party requirements, you can give security instead of insuring. Can’t remember the figure, but it will probably be lower than you think, perhaps under £1m?
That said, if it was a toss up of insuring TPL for a few hundred/thousand a year or giving security over £1m, I know which I would prefer!
For supercars, it wouldn’t have thought it would be the TPL that costs a lot.