Question: Which beacon is located 2.5 miles south of Hemel Hempstead, Hertfordshire?
Context: Hemel Hempstead Swimming Club is a swimming club based in Hemel Hempstead, Hertfordshire, United Kingdom and was established in 1913.  Hemel Hempstead Swimming Club (HHSC) ranks as one of Hertfordshire's premier clubs.  The Club has in excess of 220 members from age 5 to Masters Level and their swimmers compete regularly in a wide range of competitions. Watford General Hospital is a 521-bed acute District General Hospital situated on Vicarage Road, Watford, Hertfordshire.  It is operated by West Hertfordshire Hospitals NHS Trust together with Hemel Hempstead General Hospital in Hemel Hempstead and the St Albans City Hospital in St Albans. Kabekona Lake is a clear lake in northern Minnesota, United States, located 2.5 miles south of Laporte in Hubbard County.  Kabekona has a surface area of 2252 acre and a maximum depth of 133 feet (41 m).  Kabekona ("Gabekana" in the Ojibwe language) means "End of the Trail" to dissuade others from venturing to the very clear lake.  Kabekona is the 2nd clearest lake in Minnesota.  This spring-fed lake is known for its loon birdlife.  At one point it was thought to be the source of the Mississippi River.  Among the most popular fishes in this lake are yellow perch, bass, northern pike, and walleye.  Kabekona Lake was formerly known as "Friendship Springs". The Nickey line (also known as the Harpenden to Hemel Hempstead branch railway) is a disused railway that once linked the towns of Hemel Hempstead and, initially, Luton but later Harpenden via Redbourn, in Hertfordshire, England.  The course of most of the railway has been redeveloped as a cycle and walking path, and is part of the Oxford to Welwyn Garden City route of the National Cycle Network.  It is approximately nine miles (14 km) long. The Bovingdon stack is a section of airspace to the north west of London where inbound planes to London Heathrow Airport, which is 20 miles (30 km) to the south, are held.  It is a busy example of a hold.  It extends above the village of Bovingdon and the town of Chesham, and requires the VOR navigational beacon BNN which is situated on the former RAF Bovingdon airfield.  At busy times on a clear day a dozen planes may be circling overhead.  Other holding patterns serving Heathrow are at Biggin Hill, Kent (BIG - SE Arrivals), Lambourne, Essex (LAM - NE Arrivals) and Ockham, Surrey (OCK - SW Arrivals), where inbound aircraft will normally use the pattern closest to their arrival route.  They can be visualised as invisible helter skelters in the sky. Hemel Hempstead railway station is on the West Coast Main Line, on the western edge of the town of Hemel Hempstead, Hertfordshire, England.  The station is 24+1/2 mi north-west of London Euston on the West Coast Main Line.  Hemel Hempstead is managed by London Midland and all train services are operated by London Midland and Southern. Royal Air Force Bovingdon or more simply RAF Bovingdon is a former Royal Air Force station located near the village of Bovingdon, 2.5 mi south of Hemel Hempstead, Hertfordshire and 2.5 mi southeast of Berkhamsted, Hertfordshire, England. The Hemel Storm are a basketball team currently competing in Division 1 of the English National Basketball League.  The team is based in Hemel Hempstead, Hertfordshire and play their home games in the Hemel Hempstead Sportspace facility.  Over their short history, the club has been based in several locations across Hertfordshire and North London, before settling into their current, long-term home in 2010. Gadebridge is a district of Hemel Hempstead in Hertfordshire, UK, located north west of Hemel Hempstead old town.  It was developed from land that once formed part of Gadebridge House in the 1960s and centres on the Rossgate shopping parade.  Gadebridge Park is the largest green space in Hemel Hempstead.  A major Roman villa was discovered here at the time of its development (Gadebridge Park Roman Villa).  The population of the Dacorum ward (including Piccotts End) at the 2011 Census was 5,655. Hemel Hempstead Town Football Club is a semi-professional football club based in Hemel Hempstead, Hertfordshire, England.  Affiliated to the Hertfordshire County Football Association, they are currently members of the National League South, the sixth tier of English football, and play at Vauxhall Road.
Answer: the VOR navigational beacon BNN