Dave R, I did start looking at Doc 4444 Ch 5, 8168 Ch 7 and a few others but became quickly bored when I was trying to figure whether Liverpool was a 4E or 4F with the associated 107.5m or 137m protected area..
Lets look at this another way:
If you landed and were told to vacate and hold at F, where would you stop? Would you be mindful of Tenerife?
Equally, if you taxied prior to departure and were told to hold at F, where would you hold? Would it be the runway side of the line?
As far as I can see the UK hasn't filed any differences against ICAO SARPs here so I think it reasonable that the world expects us to be runway vacated once we have crossed the line; in other words we are not runway vacated until we have crossed the line. Consequently, the sensible thing to do would be to cross the line unless you have been specifically instructed to hold short and in this circumstance you still occupy the runway environment.