LJ,
Just to go one step further from Gonzo (don't ya miss me mate?), the STAR clearance limit is the beacon for the hold. Unless otherwise instructed, a/c will enter the holds. Aircraft are then sequenced from the holds onto final approach by the Directors. At Heathrow, the pilots don't start the approaches themselves. There are published procedures in AIP that Tori mentioned, however these would only be used in the event of an a/c in the hold who has experienced a radio failure(or a total Radar failure, in which case it would be kinda interesting to see what happens).
Hope this helps!