This topic also opens up the whole subject of conditional clearances as well as the use of stop bars. I personally never issue a conditional clearnace if I am using stop bars but I appreciate that the traffic levels at my airfield are nothing like those at EGLL/KK. In my mind, stop bars are there for the protection of both parties and I would always expect an aircraft/vehicle to query a line up or crossing instruction when a red stop bar is displayed. If ATC are routinely instructing an aircraft to cross a red stop bar then the whole impact of these devices becomes severely degraded.
As an aside, ATSIN 41 requires ATS providers to "review local procedures and practices relating to the use of conditional clearances in order to satisfy themselves that the risks of such operations are adequately controlled" I would suggest that the use of stop bars in this situation is one area that should be considered.