If ATC have the Red Stop Bar lit and you cross that - it's an incursion.
No lit stop bar - provided your wake categorey is OK - No probs.
Every year NATS do a reaction time survey so every second counts.
If you are driving a 747 and are sure LHR Ops have a clean runway why not see have far up the arse of the departing you can get
We could start a book on this
Sir George Cayley