The LHR-SIN gave the option to 'Buy a ticket'. But that option was not given for the BKK-LHR. Either the flights are fully booked (unlikely) or AF has already made schedule changes but will not notifiy me until two weeks before departure.
Rather than consulting the timetable, you would be best advised to buy tickets using the booking tool.
A timetable is not the place one goes directly to buy a ticket, so you cannot determine that tickets are not available for purchase just because you are not provided with a link to buy.
The booking tool is selling tickets for LHR-SIN in late August and BKK-LHR in early September.