I've had further thoughts about the Volage and wonder if your website identification of the ship built in 1869 is correct. That does seem rather late for the plaque, and looking at the print it seems to have only twelve guns on each side and doesn't look steel hulled. The previous Volage was built in Portsmouth in 1825 and was a 28-gun ship. This ties up much better with the pictured vessel, allowing for the odd gun or two facing fore and aft. This vessel was involved in a Battle with China (the Battle of Chuanbi) in the first Opium War of 1839-42. I just think this sort of date is much more likely for the plaque.
I'm always very happy to hear from anyone who can improve the quality of information on this site: firstname.lastname@example.org