Kettles as we know them today evolved alongside tea drinking.
Tea came to Britain in the second half of the 17th century but due to the expense of shipping it from China it was a luxury only afforded by the rich. The cost of tea steadily declined during the 18th & 19th centuries after the removal of importing monopolies opened up trade with the Orient.
By the time Victoria was firmly seated on her throne, tea was served above and below stairs.
From the first, kettles were an indispensable item of tea-making equipment. Originally, the word kettle was used for any flat-bottomed, lidded pan. Before tea became the national drink, water had always been heated in large pots hung over an open fire. This was not suitable for boiling the small amounts of water required for tea drinking. Water kettles were invented complete with lids to ensure they heated quickly and spouts for pouring and letting out steam to prevent them boiling over.
Initially when tea was expensive and brought only from China, those that could afford it, could also afford to have their kettles made from silver. These were usually small enough to sit on the tea table and due to their expense were never hung over an open fire. Instead they came with spirit burners over which they sat to keep the already boiled water hot.
Once the lower classes had access to cheaper tea, affordable, mass market kettles were needed. Copper replaced silver, because it was cheap, bright and an excellent conductor of heat. Flat bottomed copper kettles were developed to stand on the newly developed hob grate or range. Their shape was subject to experimentation.