King George III reigned from 1760 – 1814. King George IV essentially reigned from 1810 – 1830. George IV was the king during the era depicted on the series Downton Abbey. When I look at images of the kitchen of the Palace at Kew, I see things that are very familiar to the kitchen on Downtown Abbey.
George III’s formal palaces were at Windsor Castle and at Buckingham House [which is now Buckingham Palace], but he preferred the simpler lifestyle at Kew. In 1788, George III began having bouts of mental illness, and after that, he was somewhat confined there until his death. Kew palace was somewhat abandoned for about 80 years, until the time that Victoria opened it again. The kitchens were not reopened until 2012.