HMS Trafalgar, a ship of the line carrying three decks of guns, was christened in 1841, and Prince Albert was in attendance. The ship shown appears to be HMS Trincomalee, a much smaller frigate commissioned in 1917.
Diana Rigg (Duchess of Buccleuch) and Peter Bowles (Duke of Wellington) acted together fifty years earlier in the fifth season, episode 3 of The Avengers Escape in Time (1967).