I've just had to have my signature witnessed on a legal document (2x lasting powers of attorney). The only stipulations were that it must be someone over the age of 18, not a relative and not anyone who lives in the same household. My neighbour was the witness.
But... What's the point? His signature is not on the relevant records to prove that it is genuine.
My brother has had a similar experience with a life policy when trying to get his money out. He had to get his signature witnessed, but this time with a list of acceptable professions. He got a retired bank manager to sign. However the form was returned as the bank manager had put the word retired after his profession. Had he not done so I guess that the form would have been accepted without question.
If anyone was trying to defraud getting any Tom, Dick or Harry to witness and lying about their status would be the way to go.