We asked the CAA the same question a couple of years ago, ever pro active, they declined to come up with an authoritive statement so a colleague asked on here and was directed to an out of date ATSIN.
We now have a MATS Part 2 entry along the lines that;
Provide that the ADC ATCO can fully and clearly see that the entire aircraft has vacated the runway and is continuing in the same direction towards and beyond the holding point, the aircraft is considered to be "runway vacated". In LVPs, or at any other time that the ADC Atco cannot clearly & continuously see the aircraft, the aircraft must be instructed to report crossing the holding point before the runway can be considered vacated."