What is dual voltage ?

[Brains]

Something not right there, unless you are filling it to the brim.

Our 800 watt kettle with enough water in for two fairly large mugs takes 5½ min to boil.

You forgot hight!
When cooking eggs you have to add an extra minute per 1,000ft of elevation.

 

[Brains]

We have one of these kettles.
Only 650 watts, it's never tripped a bollard,
not even at an overloaded site in Germany

It boils fast enough, probably not as fast as gas, but still takes less than 5 mins.
Just make sure you never fill it to the max line, as water goes everywhere.

Next time I'm in the van, I'll measure a half litre of water, fill both kettles and see how many seconds difference there is.