Actually I worked with with an International Company based in, so we had many Brit's in our office in Chicago. This is for what I understand.
Mr. A. O. St. John pronounces his name as well as family members as Sin Gin, however they spell it out fully as Seint John. This is according to them while submitting for a patent, and it had to be legal. They are very adamant about this issue.
Back to when I was working in Chicago, North England calls it St. John, while in and around Oxford, it is Sin Jin.