From the British Library
We’re making some updates to our network, which means you won’t be able to use our website on Sunday 19 September between 08:00 and 11:00.
From The National Archives (TNA)
“migrating our website to new servers”, “It is possible that some things may not look or work exactly the same as they did before.”
LAC please note, neither anticipates the site being unavailable for days. The Brits leave that to rail service!