Hi Jonah,
The provisional flight times for the main tour are:
You'll depart from Heathrow at 14.45hrs and arrive in Phnom Penh at 13.15hrs the next day. On your return journey, you will depart from Luang Prabang at 19.10hrs and arrive back in Heathrow at 07.40hrs.
If you decide to extend your stay with the Vietnam add-on, the provisional flight times are:
At the end of the tour, instead of flying back to the UK, you'll depart from Luang Prabang at 19.10hrs and arrive in Hanoi at 20.10hrs. On your return journey, you will depart Hanoi at 01.10hrs and arrive back in Heathrow at 07.40hrs.
I hope this helps.
Kindest regards,
Monty