I'm looking at First LHR-SYD next month. The difference between the full flexible and discounted fare is really substantial. Am I correct in thinking:

F - All changes and cancellation allowed
A - no cancellation allowed and changeable for £200 each time?

OR is it more complicated than that (probably)?

My plans are very flexible, but I'd like the option to change the return leg if Sydney in the winter proves to be divine.

Thank you wise and clever FT