That is a 6th or 7th freedom right (I forget which, they are rarely provided). Fifth freedom is far more limited: it is the right to sell tickets between 2 foreign airports on flights that originate or terminate in the home country. So BA can sell BKK-SYD only for flights that orginate and terminate in the UK; BA cannot operate FRA-BKK-SYD even if it had the rights for FRA-BKK. BA also couldn't open a base at BKK, station planes there, and have flights that just run BKK-SYD-BKK and not have the same plane and flight number go to/from LHR.