1. it is a cultural thing, many are doing it 2. they have very powerful kettles

You vastly underestimate the consumption of tea in the United Kingdom. It's funny that America is almost the only country that does plug-in drip coffeemakers, though, since most people that follow "their shows" in the United States probably would be much less likely to trip a circuit suddenly.

Streaming probably dented it, but likely so did satellite and cable.

For much of my childhood there were only four (and briefly before the boom, five) TV channels. Major events like football would have a significant proportion of UK households all watching, and in those breaks a signficiant fraction would switch on an extra 1.5-2 kW device.

If the average household was four people, and 50% of households used a kettle in the same ad break, that would've been a surge of around 60e6*0.25*0.5*2kW = 15 GW in a nation whose mean load is about 40 GW.