I think that the youtube posted is rather biased against BA and has edited the tape to suit his own agenda.
Although there was a delay in the cabin being ready (hence the phonecall from the cabin), the pilot declared fully ready before he was cleared to line up. It is not uncommon to complete the BTOC whilst lining up (some aircraft that I have flown needed to be lined up before it could be completed). While he was completing this checklist the controller launched into a 16 second admonishment. When the pilot said 'standby' he was asking her to allow him to complete the BTOC uninterrupted, a request that she ignored. Had the BA aircraft been made to vacate the runway it would have caused a much longer delay than allowing him to take-off.
I realise that BA bashing is great sport, so I don't expect to have many supporters.