The date of the Base Captain's interview (known as a 'CPI' or Career Progression Interview) is the date you are added to the Command List. That list determines the order you go on the course.
This was true, but is now only partially true.
The list is currently more than a year long (CPI date to start of command course date) and with no command courses planned for a few months, it is static. Fine.
However... There has been a queue jumping system in place, whereby you can leap out of that waiting list and into a course (leaving others waiting for their offer) by accepting a non-UK contract. This has been called the "Accelerated Command Opportunity".
The last (Madrid) accelerated command courses have now run, but with a new French contract being negotiated I imagine that there will be more acceleration available, leaving those languishing on the list the opportunity to remain as a First Officer for
even longer.