M25
The road which encircles London.
Most people regard it as a useful means of speeding their commute to the office and bemoan, ad nauseam, it's long queues.
Gentlemen regard it, in the same way as the Thames and the Watford Gap, a useful geographical barrier.
The Watfortd gap is the first junction on the main motorway heading north from London.
Rule 1: NOTHING OF ANY IMPORTANCE HAPPENS NORTH OF THE WATFORD GAP.
The Thames, as even the most cursory perusal of the A to Z will reveal, separates North and South London.
Rule 2: NOTHING OF ANY IMPORTANCE HAPPENS SOUTH OF THE RIVER.
One of the Chaps refuses to visit his brother in Sydney because it counts as South of the River.
Finally, the M25 rings London giving rise to the final rule...
Rule 3: IF YOU HAVE REACHED THE M25 IT IS TIME TO TURN BACK.
These rules cover everything you need to know to survive a visit to England.