This is where the fbh timer comes into its own.
Mine is set to switch on the fbh (with the interior fan at Nr 1 and the distribution control set to screen) 30 minutes before I leave the house.
No matter how much frost or snow I get a warm engine and warm cabin.
When I do start the engine I switch the heated seats on - but normally there is no need for using the heated front screen or even the rear screen demister.
Auto box is of course a bit reluctant to change up when cold so I just take it easy for the first couple of miles then all is OK.
AFAIK the fbh burns around 250ml an hour when going full blast so my half hour of heat is costing me about 40p I suppose - but I have no need of ice scrapers or sprays - and I stay much warmer.
Of course it will have a negative effect on the mpg - but I don't mind.
It was a hard frost again this morning and it's snowing heavy at the moment so I remember all the scraping I've done in the past and just smile.
The timers come up sometimes on eBay or can be found in scrapped Rover 75s or MGZTs - lots of them around.
Singvogel.