I have some knowledge of the situation. Much that has been surmised is correct.
(1) The Train Operator hauling the train is held responsible for safety of the train and those on it, irrespective of who the organiser is or who provides the rolling stock or stewards. Should someone hang out of the window (an activity which ORR regards as unsafe) then it is mainly the Train Operator that will be pursued. Although the organisers/providers of stewards may also have questions to answer.
(2) As others have noted, West Coast Railway Company recently received an Improvement Notice related to the need to prevent people hanging out of windows on their trains.
(3) H&H hauled a Retro tour recently where someone was filmed hanging out of a window and the video put on Facebook. ORR inspectors saw the footage and have been asking questions of H&H- not surprising, as they are being consistent with their action on WCRC. The matter of the forthcoming UKR tour hauled by H&H was discussed. It was on reflection decided by H&H that given the booked traction and stock, there was a high risk of someone doing the same (hanging out of windows). So H&H, in discussion with UKR, took the responsible action of postponing the tour until physical measures (window bars) can be installed to prevent people hanging out.
(4) H&H recognise that whilst this is only a very small minority of passengers behaving irresponsibly, equally it is a small minority who appear determined to hang out of windows come what may. Reality is, the best stewarding in the world cannot prevent someone who is properly determined. Best case if this happened again, H&H suffer formal action from ORR, worst case someone doing this is killed. ORR have clearly decided to call time on the activity of hanging out of windows, and I suggest that those who don't like this direction of travel to take it up with ORR- not the train operators or tour operators. Good luck though, as hanging out of windows was basically stamped on following the Twerton fatality- remember, the RAIB will challenge ORR if preventative measures are not put in place to prevent such situations recurring.
OK, a bit of a hard truth now. It's (informally) pretty well-known that there is a minority of "enthusiasts" who like to "flail" out of the windows, and for whom that is a big part of the experience.
Well, like it or not, it is the action of such people that have led to this situation.
As with football grounds going all-seating due to the minority of "fans" causing trouble at matches, expect to see charter operators increasingly using only stock with window bars on to prevent that minority ruining it for the others.
TPO