For family history researchers, the greatest negative of this downtime will be lack of access to the Library's Catalogue and free Roman Catholic Registers database. Regarding the latter, FindMyPast's and Ancestry's indexes and links to the latter will also be impacted, with transcriptions available but no images.
Online services are expected to be functioning again during the afternoon.
UPDATE, 17 October, 18:50pm: Full service appears to have resumed.