Question

why do we british preserve official documents

Answer 1

its very important for our future generation to now about them

Answer 2

$\large{\boxed{\red{\textbf{ Thanks\:For\:Ansking}}}}$

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.