I do that too, in my TD5 Disco, but the User Manual page 113 says not to!
It says ...
WARMING-UP
DO NOTwarm-up the engine by allowing it to idle at a slow speed."
That would worry you huh?
But it gives the game away in the next sentence which starts
In the interests of fuel economy, it is advisable to drive the vehicle straight away
I reckon the same as Slob here.
In the interests of raod safety and comfort, on a frosty morning get the bugger running, put the heater fans on full blast with the temp knobs full up, and go for a second cup of coffee while the car is made safe to drive (like you can see out the windows) and confortable.
Warming up gently is probably better for the engine than start-up and drive away, especially in a diesel.
That's what I think anyway, but what do I know?
CharlesY