Via Bruce Schneier, apparently the physical security of British nuclear weapons until around 1998 consisted of, essentially, a bicycle key:
To arm the weapons you just open a panel held by two captive screws - like a battery cover on a radio - using a thumbnail or a coin.
Inside are the arming switch and a series of dials which you can turn with an Allen key to select high yield or low yield, air burst or groundburst and other parameters.
The Bomb is actually armed by inserting a bicycle lock key into the arming switch and turning it through 90 degrees. There is no code which needs to be entered or dual key system to prevent a rogue individual from arming the Bomb.
Oh. Well. Of course. Why use a hard-to-forge sequence of letters and numbers like the U.S. or U.S.S.R. when a little key will do?
So what prevented an accidental (or deliberate) British detonation until Tony Blair fixed the problem? Why, tradition, of course, what what!
The Royal Navy argued that officers of the Royal Navy as the Senior Service could be trusted: "It would be invidious to suggest... that Senior Service officers may, in difficult circumstances, act in defiance of their clear orders."
(Insert nervous laughter here.)