The video shows you entering the box behind a moving vehicle that then stops beyond the box leaving you with insufficient room to exit the box. As far as I can see the contravention is made out.
This is a well-known location for people getting PCNs and appeared fairly regularly in the now-defunct Pepipoo forum. 7 seconds is unlikely to be regarded as de minimis, certainly not by TfL, and I doubt by an adjudicator at London Tribunals.
Can you please post-up the PCN as already requested. Also it might be prudent to find out when/if they have sent a response to your representations. Can you please post your reps as well.
There are flaws in TfL management of the enforcement process, but people more up-to-speed on this aspect should be contributing soon, but please help us to help you by posting all the documents. All you have to redact is name and address.
The extract from the TfL PCN status enquiry shows reps rejected. The amount showing is £160 which is the full PCN penalty. This will increase by 50% if you don't pay the £160 when they serve a Charge Certificate making the amount £240. You could register an appeal at London Tribunals before the CC is served. However, I don't think you have a winning case, to be honest, but see what the others say.