What's the most British way of saying it's cold outside?

For me it is the Met Office: Yellow Ice and Snow warnings.

You just know it will be cold when we are expecting yellow ice or yellow snow.

OverlyAnxious said:

Elysium said:

Binky1234 said:

It's cold enough to:
Freeze the balls of a brass monkey.

Yes this one!!! 
what the heck is a brass monkey anyhoo?

It was a cannon ball storage rack on a ship. 

The cold weather made the brass contract, so the balls no longer fitted in the rack and fell off. 

Not ideal to have cannon balls rolling around your ship during a battle at sea! 

This story is a myth. This story has been discredited by the U.S. Department of the Navy,etymologist Michael Quinion, and the Oxford English Dictionary (OED).
They give five main reasons:
    The OED does not record the term "monkey" or "brass monkey" being used in this way.
    The purported method of storage of cannonballs ("round shot") is simply false. The shot was not stored on deck continuously on the off-chance that the ship might go into battle. Indeed, decks were kept as clear as possible.
    Furthermore, such a method of storage would result in shot rolling around on deck and causing a hazard in high seas. The shot was stored on the gun or spar decks, in shot racks—longitudinal wooden planks with holes bored into them, known as shot garlands in the Royal Navy, into which round shot was inserted for ready use by the gun crew.
    Shot was not left exposed to the elements where it could rust. Such rust could lead to the ball not flying true or jamming in the barrel and exploding the gun. Indeed, gunners would attempt to remove as many imperfections as possible from the surfaces of balls.
    The physics does not stand up to scrutiny. The contraction of both balls and plate over the range of temperatures involved would not be particularly large. The effect claimed possibly could be reproduced under laboratory conditions with objects engineered to a high precision for this purpose, but it is unlikely it would ever have occurred in real life aboard a warship.
The phrase is most likely a humorous reference to emphasize how cold it is.