What you write is logical, but the advice of Peter Rodger, chief examiner of the IAM, is that it can be meant and interpreted differently and that's more than a bit of a problem. So unless it's obvious from something else, such as the flashing vehicle slowing, better to interpret a flash as a warning and use other means to signal drivers out. For instance on the motorway, as on a track day, a left indicator can signal another driver to pass you.