It has absolutely nothing to do with use of transformers. Sockets in US and in UK are fused at approximately 13Amps for safety reasons. And 110V at 13amps is 1430W, while 230V at 13 amps is 2990W. You can make a kettle which uses only 1430W of power, but it would take much longer to boil water than a 3kW kettle would. You could raise the voltage to any number you like,but if the socket is fused at 13A you are not going to draw any more power out.