Apparently an urban myth.Ah, thank you, @SandboxGeneral; while I knew the joke - it is mentioned rather a lot on these threads, - an understood what it meant, I had no idea as to the background, as all I knew that it was a joke with a specific American cultural context.
Thanks very much for posting the background & clarification - actually, I find this sort of thing absolutely fascinating.
That is another story I wasn't aware of.
https://en.wikipedia.org/wiki/London_Bridge_(Lake_Havasu_City)