The "narrow" roads in the UK are at best a lane and 1/2 wide. It's actually pretty comical, many of the country lanes have hedges alongside the road and the hedges have a groove cut into them, which is precisely at the height of the average exterior mirror. When @Argon says that they are narrow, he is employing the understated humor, for which the Brits are world renown.
The last time that I went to England, I was with a large group. Since I just happened to not be drinking that night, I had to drive us back to our hotel in a 20-passenger, right hand drive turbo-diesel bus. I had to drive us down some of these "narrow" roads at night ... with a manual transmission ... with nineteen drunks in the back! It was terrifying!!! Thankfully, on balance, Brits are much better drivers than the average American and they kept their distance from me. If not for their driving skill, complete disaster may not have been averted.
Lesson learned ... when in the UK, always drink copious quantities of alcohol and you'll not be volunteered to drive the bus.