I have read a thread indicating on BA the chances are minimal one can board an earlier flight to a destination. I'm curious does the same apply on GB Air which is codeshare with BA?

I'm researching a flight from LGW to TFS at 19:00 but an earlier one is available at 14:45? As a ET class of service is this something they will honour for a fee or simply tell you to wait until your scheduled departure?

As others have indicated - frequent traveler of US carriers so never an issue on this side of the pond - if seat is available and airline isn't selling it they prefer to get you (stand-by without luggage) to your destination and possibly sell your seat later.