bottie,
Many airfields do insist on 3rd party insurance, and it usually is mentioned in Pooleys - just take a look! As for how they enforce that rule, well, I have no idea...
As for why it's not compulsary, I'd guess that's because car accidents involving 3rd parties are quite common, whereas the vast majority of aircraft accidents don't involve 3rd parties - landing accidents are usually confined to the runway, and I believe they're the most common type of accident. I'm not saying that justifies the law, but it may be the reason?
Personally, I wouldn't want to invest the kind of money that an aircraft costs without insuring it.
FFF
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