Modern Diesel engines are so efficient that they don't waste much energy/fuel as heat.
So they take a long time to warm up from cold, hence the need for additional heating.
The Webasto units are small diesel powered additional pre heaters that can be used to warm a car up and defrost it, (on a timer or remote control) so the vehicle is warm and defrosted when it's needed.
They are mostly used in countries much colder than England.
I don't know if they are still around, but Kenlowe, the people who used to sell electric radiator fans in the days before they were commonplace, used to make a 3kw electric pre-heater that you plumb into the appropriate hose on the cooling system. This is controlled by a clock timer to ensure your engine is nice n warm before you drive off. Had one fitted to our Land Rover years ago. Not cheap, but cheaper than the diesel alternative. Ideally the plug that powers it should be situated so that it unplugs itself should you happen to forget to unplug before driving off. Failure to observe this warning will result in "the dissipation of energy in the form of heat and light" as my old science master used to say. My wife knows this to be true.
'56 Multispace 1.6HDi - Iron Grey
Don´t know if these guys sell in the UK, but they are fitted as standard to most petrol cars in Finland
Warms the engine before starting as well as the interior
My car also came with a winter shield for the radiator, which helps the engine run at a higher temp. and is a bit tidier than a bit of cardboard