Might as well throw in my "you're fine" as well, but to answer "what was I supposed to do?" In future you should be moving forward to a position ready to make the turn (i.e. where the van was sitting) after the vehicle in front has moved*. That would remove all ambiguity as to whether you cleared the stop line when the light was on green/red (and also mean you are able to clear the junction quicker in scenarios such as your video).
* assuming your exit is clear - the highway code entry (174) for box junctions does not prohibit stopping in the junction, it only states you MUST NOT enter if your exit is not clear, and also include the advice that "you may enter the box and wait when you want to turn right, and are only stopped from doing so by oncoming traffic, or by other vehicles waiting to turn right."