"Whereabouts does the Thrush lay at Spithead? Near the Canopus?"
Spithead is a sheltered part of the Solent where ships may lie at anchor.
H.M.S. Canopus began life as the French-owned ship Franklin which was launched in 1797. Following its capture at the Battle of the Nile in 1798, it went into service with the Royal Navy, where it remained until 1887.
Sir Francis Austen, Jane Austen's brother, served on H.M.S. Canopus, and was its commander at the Battle of San Domingo in 1806.