Hi,
Sorry to say it... but if you think you will make £15k a year as an FI you're living in dreamland... I wont make anywhere near half that........ you'll be lucky to fly more than 500-600 hours a year.. most places pay around £12 an hour or so... if you're lucky you might get a retainer as well.
I dont have a clue what instructors make at higher level.. eg teaching at CAP509 or Examiners etc etc...