It's not.
It's a joke mail that's been going round.

Rule 229 says
Before you set off

* you MUST be able to see, so clear all snow and ice from all your windows
* you MUST ensure that lights are clean and number plates are clearly visible and legible
* make sure the mirrors are clear and the windows are demisted thoroughly
* remove all snow that might fall off into the path of other road users
* check your planned route is clear of delays and that no further snowfalls or severe weather are predicted

But it's not an endorsable law.