Around 133,000 British nationals visited Cambodia in 2014. Most visits are trouble-free but there have been reports of assaults and armed robberies against foreigners. See Crime
National elections took place on 28 July 2013 in a mostly peaceful environment. The main opposition party disputed the results and this led to a number of protests, most of which took place in Phnom Penh. While most demonstrations were peaceful, some clashes occurred. The most severe took place on 3 January 2014 when 5 protestors died.
On 22 July 2014, a political agreement was reached by leaders of the ruling party and the opposition, but it is possible that further protests may take place. You should avoid all public gatherings and monitor local media closely.
There is a low threat from terrorism. See Terrorism
Take out comprehensive travel and medical insurance before you travel.