An aircraft is not a 'public place', therefore the operators are within their (UK Law) right to prohibit photography
This was a special flight -- being the first ever A350 departure from a US airport -- filled exclusively with invited journalists, influencers and other VIPs. There were no regular passengers on board.
The QR flight attendant asking one of the journalists to "switch off the camera" did not go very well with the group. No doubt the
PR damage control is continuing.