When a last name contains "Saint", is it ever spelled out, or is it always "St."?

In English Law, in England, you can call yourself whatever you want, providing (I don't know the exact wording) it is not for the purpose of deception in connection with an illegal act).

So when it comes to official documents you can write your name as YOU want, again providing there is no criminal intent. Leaving aside officially changing your name by deed poll ,which means all documents that had your previous name are legally binding and acceptable with the new name even though they are not physically changed or re-issued.

Perhaps a more complete answer can be found by a little study here, see what people actually wrote on the documents https://www.nationalarchives.gov.uk/