Yes you are correct but the FTSE could drop say, 1000 points before it goes back up to 5500 or even 5600. Just as you may gain £1 per point above 5500 (after you cover the spread) you will also lose £1 per point below 5500. If it were to drop 1000 you would be £1000 in the hole. If you do not have those funds on your account IG will ask you to top up or close your position.
If you have a limit stop set your position will close automatically at 5600 yes.
Hope this was helpful.