The vast majority of couples in the U.K. are happy. Researchers from the Institute for Social and Economic Research at Essex University asked married and cohabiting couples to rate their happiness on a seven-point scale from the lowest score of 'extremely unhappy,' to the highest point of 'perfect.' 90% of married women and 93% of married men said they were happy in their relationships with the figures being 88% and 92% respectively for cohabiting couples. The happiest couples were those in which both partners were educated to degree level, had no children, that had been together for less than five years and where the man was employed. After taking into account a variety of factors including age, gender, number of children, length of relationship, employment status and education married couples were found to be happier than cohabiting ones.
You can find out more about this research here.