Extract from UKCAA doc CAP 413 Radiotelephony Manual.
Conditional phrases will not be used for movements affecting the active runway(s), except when the aircraft or vehicles concerned are seen by the controller and pilot.
Conditional clearances are to relate to one movement only and, in the case of landing traffic, this must be the first aircraft on approach. A conditional instruction shall be given as follows:
a) callsign;
b) the condition;
c) identification of subject of the condition;
d) the instruction.
e.g. “Fastair 345 after the landing DC9, line up”
Thus I interpret this as not to cross the line until the landing traffic has passed you. The other argument for this is that if you do move forward you may infringe the obstacle clearance zone for landing traffic.