From the Museum Of Flight Website:
"The world's first and most successful vertical take off and landing jet to enter service, the Harrier has matured into a formidable multi-role strike aircraft. After a series of concept development aircraft, the first of which hovered untethered for the first time in 1961, the pre-production Harrier first flew in 1966. XV277 was the second of these and is the oldest Harrier in existence. The Harrier has created a niche for itself as an affordable aircraft carrier based attack fighter, although developed for the RAF for use from dispersed sites; five other countries' Harriers are ship based. XV277 led a life of trials and testing, modified to GR.3 standard from the basic GR.1, it never saw operational service in either of the British armed forces that operate the type. The Fleet Air Arm at Yeovilton used it as a ground instructional airframe before being sold to a private collector. It was purchased by the Museum of Flight in 2000. "
Other site info: The Royal Navy markings were picked up in the days when it was a ground instructional airframe. It was operated during its flying career by the manufacturer.