Since your original date is in en-GB culture, you can create a CultureInfo object and parse your DateTime naturally.
string date = "28/08/2012";
System.Globalization.CultureInfo ci = System.Globalization.CultureInfo.CreateSpecificCulture("en-GB");
Convert.ToDateTime(date,ci.DateTimeFormat).ToString("d");//short date pattern
(OR)
DateTime.Parse(date,ci.DateTimeFormat).ToString("d");