A kangaroo loose
In the top paddock.

See also:
Madder than a wasp in a jam jar.
(Stop, look, listen - Pheeeel. ...most turnin', stormin' sound of soul, Fri 12 Jan 2007, 13:29, archived)

Madder than...
...a goose on stilts
...a sackful of badgers
...a barrel of monkeys
...an ape in wellies
(rnuk http://flickr.com/rnuk, Fri 12 Jan 2007, 13:31, archived)

' He's a couple of stops short of Dagenham' is my favourite one of those.

( Wicca'd Witch needs ideas for a new sig on..., Fri 12 Jan 2007, 13:31, archived)

'Barking'
?
( Cap'n Tallbeard, Fri 12 Jan 2007, 13:34, archived)

We tend to use "he's just past Upton Park".
Or shorten it to West Ham, Upton or East Ham depending on how insane.
( The Teviot Moose major 5th., Fri 12 Jan 2007, 13:34, archived)

My brother used to say
"I'm off for a Duncan Ferguson" when he was off for a shit.
(rnuk http://flickr.com/rnuk, Fri 12 Jan 2007, 13:37, archived)

Mornington
Crescent
(Stop, look, listen - Pheeeel. ...most turnin', stormin' sound of soul, Fri 12 Jan 2007, 13:35, archived)