Is forced marriage human trafficking or cultural variation?

A British author asks that question: Forced Marriages Dishonour Britain

"If white British women were being forced to marry men they had never met, in a country they had never visited, there would be a national outcry. We would call it trafficking.
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"In the UK, forced marriage affects mainly women and girls from South Asia as well as smaller numbers from Sudan, Turkey, Afghanistan, Eritrea, Armenia, Somalia and the Irish traveller community.  FCO dealt with over 1,735 cases in 2010, whereas estimates from feminist organisations dealing with the issue range from 450 to 1,000 victims a year. The difficulty of collecting data is compounded by the fact that the line between an arranged marriage and a forced one is not always clear."