That's not the test, unfortunately, it's entering so you have to stop within the box due to stationary vehicles.
You may be able to argue the other car was obliged to give way and you had the right to assume the exit would be clear and he nipped in when it was too late to safely stop before the junction.
You can also argue the location stated on the PCN isn't specific enough as there's more than one box junction along Plumstead Common Road.
Here is a case for your reference that supports MrChips' advice:
Case ref 2230020427
Appellant Tyrrell Jean
Authority LB of Barking and Dagenham
Date & time 18 Dec 2022 09:27:00
Location North Street
Decision Date 20 Feb 2023
Adjudicator John Lane
Decision Appeal allowed
The issue of this appeal is whether the said vehicle entered and stopped within the box junction there owing to the presence of another stationary vehicle. It is a contravention if a person causes their vehicle to enter a box junction so that the vehicle has to stop within the box junction due to the presence of stationary vehicles.
To stop means to come to a stand as in the course of a journey, to halt or to cease moving.
The contravention does not apply to any person
a) who causes a vehicle to enter the box junction (other than a box junction at a roundabout) for the purpose of turning right: and
b) stops it within the box junction for so long as it is prevented from completing the right turn by oncoming vehicles or other vehicles which are stationary whilst waiting to complete a right turn.
The appellant has stated the vehicle to his left came out of the side road and effectively cut up his vehicle.
I have considered fully the representations of both parties and I have examined carefully the video evidence provided by the local authority and have considered the local authority’s representations.
I agree with the appellant’s version of events, however. I find that the appellant’s vehicle would have cleared the box junction had it not been cut up by the vehicle to the left. The fact that the vehicle to the left did not clear the box junction does not affect my judgment.
For others to help you faster on this forum, please share the PCN number and the number plate. Also possibly the Google Street View URL of the location of your PCN.
This suggestion is from my personal experience on the forum and is also in-line with the guidance
