Fully flexible means fully refundable.

It is free to cancel online, but the likelihood of being able to do that are less than a herd of unicorns stampeding into my office in the next 27 minutes.

There is a £15 charge to cancel on the phone.

You can "no show" and cancel after the flight. Or simply re-book after the flight was meant to be. The booking remains live for about 3 days in the system.