The lowest rank likely to command a Royal Navy vessel

What is the lowest Royal Navy rank that actually commands a ship or boat?

Usually lieutenant commander, although lieutenants can sometimes receive command.

Command depends on the vessel and appointment rather than the title captain alone. A lieutenant may find themselves responsible for a small craft or vessel, but lieutenant commander is the more typical junior command grade. The commanding officer's appointment matters more than whether their substantive title includes captain. The distinction reinforces the difference between holding command and possessing the substantive rank of captain.