Upvote:1
You can take Heathrow to Bangkok 11:50 – 06:15+1 with Thai then get one of flights to Phnom Penh
They go out like every 2 hours or so (just watch out for AirAsia they fly from Don Mueng)...
Upvote:6
As mentioned in the comments you have very few options flying into Cambodia.
You can check Cambodia Airport site to see exactly who flies in and out of the 3 major airports in the country.
The closest major airport to Phnom Penh that one might fly into actually appears to be Ho Chi Minh in Vietnam, though to fly there you will still have to make a connection though options are more plentiful.
So if you do choose to fly to Phnom Penh the best options would be to connect through Incheon, Kuala Lumpur, Singapore, Bangkok, or possibly Ha Noi, which offers shorter layovers on the the way there but may be north of 6 hours on the way back.