Shortly after becoming airborne from a private grass airfield, all four occupants on board the aircraft heard a “thump”. Following some troubleshooting, the Pilot Flying (PF) concluded that the aircraft had suffered a bird strike and subsequently reported this to Shannon Air Traffic Control (ATC). The aircraft continued its climb to FL195 en-route to Lisbon, Portugal. Sometime later, the PF observed on his fuel gauge that the port wing tip tank was registering empty, while some fuel still remained in the starboard tip tank. As the PF was now no longer fully sure of his actual fuel contents, he decided that it would be prudent to divert to Jersey Airport. After an uneventful landing at Jersey (17.07 hrs), an inspection of the aircraft found that the entire wingtip tank and a portion of the port wing/aileron assembly were missing. Subsequently, it was determined that the aircraft had struck a tree on departure from Brittas House Airfield. There was no fire or reported injuries.