I wondered that, but times suggest otherwise, eg, Beziers timings as follows:
Southampton - Beziers
BE3915 10:35 13:35 M . . . F . . - - 27-May-11 - 09-Sep-11
Beziers - Southampton
BE3916 14:10 15:05 M . . . F . . - - 27-May-11 - 09-Sep-11
Block time of around 2 hours seems about right for that route on the Dash?