The UK Border Agency is helpful website to figure out difficult visa & passport issues. Here is some info from them about dual citizenship
(http://www.ukba.homeoffice.gov.uk/briti ... tionality/):

"When becoming a British citizen
You do not need to give up your present citizenship or nationality to become a British citizen.
Many countries will not let you have two nationalities (dual nationality). If you become a British citizen and are a national of a country which does not allow dual nationality, the authorities of that country may either regard you as having lost that nationality or may refuse to recognize your new nationality. Before you apply for British citizenship you may wish to check what your position would be with the authorities of the country of which you are a citizen."

I also found some good information about Cambodian citizenship here http://www.multiplecitizenship.com/wscl ... BODIA.html:
unfortunately "DUAL CITIZENSHIP: NOT RECOGNIZED. Exception: A Cambodian wife of a foreign national is permitted to retain her Cambodian citizenship unless required to renounce it by the laws of the husband's home country."

I would contact your nearest embassy to sort out the matter. As far as Cambodia, I thought this site with interviews from expats was quite interesting, and hopefully helpful to you: http://www.expatinterviews.com/cambodia. Here is also a link to the ministry of foreign affairs. http://www.mfaic.gov.kh/evisa/

Best of luck & congrats on your baby boy!