What is the best place in Cambodia (or around) to fly from London? I was searching for flights from London to Phnom Penh airport (PNH), but all the flights were really long with inconvenient layover times, etc.
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.