Newsnight has discovered that until the early days of the Blair government the RAF's nuclear bombs were armed by turning a bicycle lock key.
There was no other security on the Bomb itself.
While American and Russian weapons were protected by tamper-proof combination locks which could only be released if the correct code was transmitted, Britain relied on a simpler technology.
And, further on:
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.
Well, that's encouraging. I wonder if they restricted pens in the area of these nukes equipped with bicycle locks, since as this video shows you can use a pen to get a bicycle lock open: