Is it ok to use hot water to defrost your windscreen etc ?
No, NEVER use hot water to defrost your windscreen.
Never use hot water to defrost a windscreen, you’re technically correct in thinking that hot water is the quickest way to melt a build-up of ice, but never use it on a glass surface. The accumulation of ice on glass means that the temperature of the glass is extremely cold, throwing hot water onto this glass surface immediately causes a rapid temperature change and sudden expansion of the glass, almost certainly leading to cracking of the glass.
You are far safer using a scraper or a tin of De-Icer spray, accompanied by your engine running & the heater vents on hot and pointing up towards your windscreen.
Two additional points worth mentioning are, before you turn on your wipers on a frosty morning, carefully check with your fingers that the rubber part of your wiper blades aren’t stuck frozen to your windscreen, if they are, gently peel them away from the glass or wait till the De-Icer has melted the ice before switching them on, this will prevent the rubber tearing and destroying your (expensive) wiper blades.
Also, never use your wipers to clear a heavy covering of snow from your windscreen, always clear the snow first with a proper scraper, again if you don’t, you could damage the wiper mechanism or blow a fuse !