Question: W.H. Walker and Brothers was a narrowboat builder based in a small town approximately 20 mi northwest of what?

Context paragraphs:
Goshen Pass: Goshen Pass is a water gap, or gorge, in the Little North Mountain, formed by the passage of the Maury River, approximately 20 mi northwest of Lexington in Rockbridge County, Virginia.  State Route 39 traverses the pass along the banks of the Maury River.

Duncans Point: Duncans Point is a cape on the Pacific Coast of northern California in the United States.  It is located in Sonoma County at , approximately 45 mi northwest of San Francisco and approximately 20 mi west of Santa Rosa.

Southern Polytechnic State University: Southern Polytechnic State University (also called Southern Poly; abbreviated SPSU) was a public, co-educational, state university in Marietta, Georgia, United States approximately 20 mi northwest of downtown Atlanta.  Until 2015, it was an independent part of the University System of Georgia and called itself "Georgia's Technology University."

Kennesaw State University: Kennesaw State University (KSU) is a public, coeducational, research-oriented institution with the main campus located in Kennesaw, Georgia, approximately 20 mi northwest of Atlanta.  A secondary campus is located in Marietta, Georgia.  KSU also holds classes at the Cobb Galleria Centre, Dalton State College, and Dallas.  Current enrollment is over 35,000 students, making it the third largest in the state of Georgia and one of the top fifty largest universities in the United States.

Virgil C. Summer Nuclear Generating Station: The Virgil C. Summer Nuclear Generating Station occupies a site near Jenkinsville, South Carolina, in Fairfield County, South Carolina, approximately 20 mi northwest of Columbia.  The nuclear power station includes the decommissioned experimental Carolinas-Virginia Tube Reactor (CVTR) unit, just outside the site of the old town of Parr, SC.  The CVTR was a 17 MWe, heavy water reactor.  Its cooling water is supplied by the Monticello Reservoir (not to be confused with the Monticello Nuclear Generating Station in Minnesota), which is also used by a pumped storage (hydroelectric) unit.  The plant utilizes a once-through cooling system.

W.H. Walker and Brothers: W.H. Walker and Brothers was a narrowboat builder based in Rickmansworth, England. The business was established in 1905 by Harry Walker.  He leased part of Frogmoor Wharf, the Grand Union Canal, from Robert Grosvenor, 2nd Baron Ebury.  The firm thrived for most of the twentieth century, from 1905 to 1964.  They were one of the major producers of narrow boats for the English Canal Network.  The site was sold for redevelopment to Tesco in June 1989.

Rickmansworth: Rickmansworth is a small town in south-west Hertfordshire, England, situated approximately 20 mi northwest of central London and inside the perimeter of the M25 motorway.  The town is mainly to the north of the Grand Union Canal (formerly the Grand Junction Canal) and the River Colne.  The nearest large town is Watford, approximately 5 mi to the east.  Rickmansworth is the administrative seat of the Three Rivers District Council; the local authority is named from the confluence of three rivers within Rickmansworth's borders; the River Gade and the Grand Union Canal join the upper River Colne near Rickmansworth's eastern boundary and are joined by the River Chess near the town centre from where the enlarged Colne flows south to form a major tributary of the River Thames.  The town is served by the Metropolitan line of the London Underground and Chiltern Railways from London Marylebone to Aylesbury.

Coral Springs, Florida: Coral Springs, officially the City of Coral Springs, is a city in Broward County, Florida, United States, approximately 20 mi northwest of Fort Lauderdale.  As of the 2010 United States Census, the city had a population of 121,096.  It is a principal city of the Miami metropolitan area, which was home to an estimated 6,012,331 people at the 2015 census.

Franklin, Oregon: Franklin is an unincorporated community in Lane County, Oregon, United States.  It is located near Veneta, and is approximately 20 mi northwest of Eugene.

Grapevine Lake: Grapevine Lake is a reservoir located in the North Texas region, approximately 20 mi northwest of Dallas and northeast of Fort Worth.  It was impounded in 1952 by the US Army Corps of Engineers when they dammed Denton Creek, a tributary of Trinity River.

Supporting paragraphs: W.H. Walker and Brothers Rickmansworth
Answer: central London