In Series 1 Episode 1 of Ashes to Ashes, DI Alex Drake employs a whiteboard as she reasons out her situation with Shaz and Chris. Were whiteboards in general use in 1981? Is this an anachronism?
It's certainly not an anachronism, although they weren't in very wide use.
Their creation can be placed sometime in the late 1950s, although the first dry erasable marker wasn't invented until 1975.
So they were certainly around in 1981.