This is a list of the ICAO's standard phraseologies used for preparation for takeoff. I don't understand why "lining up" is (recommended to be) used as a response to a conditional line-up instruction. It can be very confusing, because it sounds like the pilot is lining up (against the ATC's intention) when he has to wait until the condition is met, like in the situation where the pilot has to wait until the arriving airplane touches down before she can taxi onto the runway to line up. I'm not a native speaker, so it might be just me. Does it sound reasonable to native speakers?
"Lining up" is not a proper response to a conditional lineup instruction;
"(condition) lining up (brief reiteration of condition)" is correct.
To use an actual example:
ATC: "RYR123 behind landing Airbus 320, line up runway 28L behind"
Pilot: "Behind landing Airbus 320, lining up runway 28L behind"
In my opinion, that leaves very little room for confusion.