The British Consulate in Cambodia is located in Sihanoukville near the Cambodian Southern coast near the Ocean Walk in. This is only an Honorary Consulate. Most Cambodians however use the British Embassy in Phnom Penh if they need to apply for a visitors visa to the UK or a marriage visa. If you are in need of a new passport or documents from home it is best to contact the British Embassy in the capital Phnom Penh for assistance.
See the listing for the British Embassy in Phnom Penh.