Question

what did British do to preserve the important official documents and letters

Answer 1

there was a letter maker who makes a copy of important documents and letter .

Answer 2

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British had preserved official documents, because they thought if they write and record everything, it will later be easier for them to study them and debate. They kept official records of what they thought was right, so that in the future, those records can serve as proofs of all the decisions that they had taken.