UK Air Passenger Duty for flights departing the UK recently increased (long haul):

Y: increased to 40 pounds (per longhaul pax)
C/F: increased to 80 pounds (per longhaul pax)

So, taxes on award tix BKK-LHR-BKK in C/F are THB 12,000 per pax, but only THB 7,000 if I depart from FRA.

So for 3 of us inC, I can save a whopping 3 x THB 5,000 = THB 15,000 or USD 430.