When your boarding pass is scanned at the CCR a big pop up will come up stating that you are in the wrong terminal. Questions will then be asked to establish how you came to access the T5 departure lounge on a T3 boarding pass as you will be considered a ghost.

Ghosts are met by HAL and asked to look at the CCTV so your image can be captured by the control centre to establish where you entered and how you became a ghost.

Of course you would just need to say you are on a flight to EDI flight at which point you will be told you are not eligible.