I'm surprised the RAIB aren't taking an interest in this (perhaps they are but as the loco was apparently re-railed and back on shed at 2am the next morning, it would give them little time). Things could easily have turned out a lot worse. While they may not carry out a full investigation, they would certainly issue some safety directives.
There appears to be both a driver and fireman on the footplate, and a signalman can also be seen in the box. Given that the only circumstances when a driver should pass a signal at danger under their own authority is when they cannot contact the signaller or when the signaller has has given authority to a shunter/pilotman, it would seem that there has been a misunderstanding in the communication between the two as there doesn't appear to be anyone acting as shunter/pilotman (unless they are out of shot).
If you want better understanding of the procedure for passing a signal at danger, this should prove interesting
http://www.rgsonline.co.uk/Rule_Book/Rule Book Modules/S - Signals/GERT8000-S5 Iss 3.pdf
My personal thoughts, which are of course purely speculative, are that the signaller doesn't appear to be unduly concerned that the loco is moving, so perhaps he believes the road to be set correctly or that the driver is only moving forward a short way. It still doesn't look like anyone has provided an indication to the driver to move off though unless this cannot be seen. It was also the driver's responsibility to check the road was set prior to the move, even more so given that the signal is on.
I work as a shunter at a preserved line and I have
never witnessed a shunt which has started without a designated shunter or signaller providing a hand signal to the driver and where there has been any doubt over the person who holds authority over the movement. Given that shunter is one of the most dangerous jobs on the railway, it is absolutely safety critical to know who is doing what.