The aircraft commander reported seeing a military fighter aircraft pass close in front of him whilst his aircraft was cruising on Airway P155 at FL210. The conflicting aircraft was later identified as one of a pair of United States Air Force F15E ‘Eagle’ aircraft diverting from RAF Lakenheath to RAF Valley. The military aircraft were both low on fuel and were diverting due to poor weather and air traffic delays at Lakenheath. They had climbed above their cleared level and one aircraft passed within 0.53 nm of the aircraft on the airway. It has not been possible to determine the vertical separation during the ‘Airprox’ encounter.