Where did the R/T phrase 'Hold short of...' originate? If it's just got into the aviation lexicon through casual use it is dangerous, because without a well-defined interpretation it can mean all things to all men. So, regardless of experience at LHR, controllers using it should not be surprised if a pilot interprets it in a manner different to what they intended.
Has the phrase migrated from the USA, where LAHSO procedures are commonplace? If so, it's dangerous to use the phrase here without the well-defined procedures and associated runway/taxyway markings required by LAHSO.