LP80

Hi, hoping for some clarity re upcoming travel, any help appreciated. I have to travel back to London for a week or so, am on a 1 year Thai language education visa. Have been told that I need to fill in a form prior to leaving otherwise validity of my visa will be voided. Having trouble working out whether I can simply fill out said form at the airport, or whether I need to go to immigration and above all what the form is even called. Thanks in advance, L