I'd like to know what would have been the most common route to travel from England to Canton, China, in 1810 and how long the journey would have taken.
The China trade routes were already established by that time, so a merchant ship, typically one classed as an East Indiaman, which carried supplies enough to make the long voyage, would sail 'around the horn' or around the Cape of Good Hope, depending on the stops. They would often stop at at India , Australia or the Sandwich Islands (Hawaii) and then proceed to China.
I found one ship, coincidentally named Canton, that has some recorded voyages from England to China, this one in 1810:
EIC voyage #8 (1810–1811) Captain George Gray acquired a letter of marque on 22 March 1810.5 He sailed from Portsmouth on 28 April, bound for China. Canton was at Penang on 5 September and Malacca on 26 September, before she arrived at Whampoa on 10 December.
So, this is about a 4-6 month trip, depending on winds and stops