A6 Diesel heating up

Is it only my Diesel A6 Avant that isn't heating up quick enough to allow switching the heating on?

2.0 engine and with temperatures like today morning 0.5deg.C I cannot turn the heating on as the engine temperature will not rise fast enough for the whole journey.
Fair enough, my daily commute with school run and to work isn't the longest one but it is surely getting the engine to a nice temperature. Soon as I turn the heating on and stop on traffic lights, engine temperature drops. I switch off the heating and drive, temperature goes back up.
If I drive for 5-10 minutes after 90 has reached and then switch the heating on, it is fine and maintain at that level.

Does anyone elses car does that?

Seen black cabs driving around with cardboard stuck behind the front grille and think I get the idea of it but not too sire about the engine safety.

Seen on internet those sticky plasters/patches with heating that you can attach to the bottom of the engine block and hard wire to your car electrics. With relay it allows to be switched on and off but should help to heat up the engine, no?

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Thermostat been replaced few years ago and even then it was the same situation.

Ever since I got the car it was like that.

Cannot compare to my previous owned vehicles as one was petrol 1.4 litre engine Fabia and second (A4 B7) had blown head gasket so cold air always and temp was up and down, even while driving on the motorway.

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