Apart from the fact a 4kW kettle would be difficult to purchase as it would overload a standard UK 13A plug you are absolutely right. 3kW is the maximum. A kettle of water takes a finite amount of energy to be brought to the boil. Whether that is provided by (say) a 2kW or 3kW kettle is irrelevant, but you’re right to point out the lower rated kettle would take proportionately longer to do it, and use the same energy in the process. Being pedantic, if they are of the same construction the higher rated kettle is slightly more efficient as the reduced time taken to boil reduces the heat loss to the surroundings by convection 
I don’t know if it’s true, but I’m sure I heard a number of years ago that the EU actually considered reducing the rating of electric kettles as part of their energy efficiency measures. Regrettably, there are few qualified engineers in government.
JS
As I recall a similar comment at work was:
‘Those of you who think you know everything are annoying those of us who do’
Meantime hopefully…our girls are gainfully employed making our tea… 
Bit of lemon drizzle or summat please…
Is it a slow Mx5 news day…? 
Back to the topic, it can be useful for the more OCD among us to create a spreadsheet listing everything in the house that might be connected to the mains!
It can also be scary.
It also helps to get a grip on the numbers if one buys a couple of power measurement devices:
One that can go between the plug and the socket (my Belkin is old and takes 0.5W and is then accurate to 0.1W)
The other monitors the mains through the meter (my Owl is also old but at least it is as accurate as the meter over a week in total kWh used, although it only shows the real-time power usage in approximately 16W steps).
It gets easier if broken down by rooms.
It gets more complicated (more OCD) if you introduce usage ie hour per day, days per week, seasonal etc
However, it only works if the sums the spreadsheet calculates add up to the same numbers as the consumption being paid for. Beware, there is a big temptation to massage the data to make it fit!
The Belkin device I have allows one to sum up the usage of (eg) a kettle over a couple of weeks; it knows how long it has been connected and gives a running estimate of the annual consumption, 180kWh in our case.
Our tumble dryer (an AEG heat pump thing) and the washing machine each use about the same in a year as our kettle, and our well insulated Miele oven about 3/5 that figure (a stark contrast to the ridiculously inefficient Howdens oven the Miele replaced!).
The biggest single usage by far used to be incandescent and fluorescent lights being left on all over the house. Now while the whole house is LEDs, some on PIR switches (halls, loos, external), the lighting still is the biggest, but only four times as much as that kettle instead of fifteen times.
Now the key point: if I add up all the tiny devices on 24/7 standby, including indicators etc on wall switches but not including t’internet etc, they come to about 100kWh per annum, almost comparable with the tumble dryer or the fridge-freezer!
An OCD example, by serendipity I saved 50kWh per annum by using an alternative system to a BTHub. With so many things on all the time they can add up to a lot and it can be worth while checking them…
There should be the option to change the smart meter display from pence per hour, to kwh. That will show you any consumption that isn’t registered on the cost screen.
Smart meters are fine, the systems we have to use to install them are not great. And can lead to very many installation issues.
There are 3 reasons to turn off appliances at night.
Safety
Save the Planet
Save money.
Saving money is ridiculous, since you only need to have one less pint and you’ve made more savings and needn’t be bothered with a on/off ritual every day.
Saving the Planet? Must make some contribution, once the Russian, Chinese etc join in.
Safety? A lot of trouble for a minimal risk.