Possible Clues:

• Shifty type, like John Dawkins
• Canadian length
• What some British poetry has
• Measure
• It's little more than the yard where you come across 'er, by the sound of it
• British standard measurement
• Sounds like the way to come across 'er in metric form
• London police get about more than the Yard
• Encountered engineers at some length
• 39.7 inches in London