It is a signal for cancellation. However, given the time of year and it's two days out I wouldn't be surprised if it was just genuinely full and slightly oversold. One reason they'll 0 out is, in the event other other flights (such as LHR-SFO) get cancelled, they can't rebooked onto this already full/oversold service.
Furthermore, a lot of UK-US flights have been offering op-ups in the past few days (see
thread) so they're trying to limit people
actually getting invol. denied boarding. All in all, nothing to worry about if I was in your position.