How a warship passes from operations to maintenance

Did sending a major warship for maintenance require unusual use of the First Sea Lord's authority?

No. Releasing an operational ship for scheduled maintenance was a normal transfer from First Sea Lord to Third Sea Lord authority.

Arranging a scheduled maintenance period did not require exceptional intervention. An operational ship sat under the First Sea Lord, then passed to the Third Sea Lord's authority for material work. Newly built ships followed the reverse path when commissioned. This division separated operational readiness from engineering responsibility inside the Admiralty. A ship could be afloat and apparently usable while still remaining under the Third Sea Lord before commissioning.