Ethel is correct and so I stand corrected - I think.
I called Direct Line (my insurers) they confirm that I can drive any vehicle not owned by me third party (in an emergency only) - but that crucially, as Ethel has stated, it must be insured by someone. I would not for example be permitted to drive Ethel to hospital in her own car if I were to come across her in the middle of nowhere, Ethel having come to some terrible misfortune, and is unconcious and incapable of consenting to my driving her car. (Though a defence of 'necessity' may be available depending on other circumstances). But this surely depends upon the conditions of use by that individual insurance company. So the only way to be entirely sure is to contact your insurer and to check your policy documents.
Regarding uninsured cars and the police impounding it though, this is new territory and it has been shown that quite commonly, the database is out-of-date. Insurers and the police are investigating ways that information be more accurate and updated more quickly. If the car I bought and insured earlier today shows up as being uninsured, the police can indeed impound my car. But not if I can subsequently show that the car was in fact insured at the relevant time. Would I have to pay to get it back? Lord knows! I tried calling the police and the chap I spoke to was clueless:-)