"Cleared to line up behind," ah, that would be, "after the departing (company, type) line up and wait (runway designator)" I presume?
And NO I would not advocate crossing ANY red stop bar without the express permission of the Controller! SAFETY is more important than EXPEDITION. The line up lighting at Heathrow is not perfect, but Controllers should be adept enough at using it to avoid the loss of too much time in the departure sequence.