It's not a contravention as you didn't have to stop in the box owing to stationary vehicles.
Kingston has form in issuing ridiculous yellow box PCNs and if they insist on taking you to the tribunal you have a good shout for costs, as the cases below show.

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Case reference 2240526422
Appellant xxxxxxxx
Authority Royal Borough of Kingston Upon Thames
VRM KK03 RJK
PCN Details
PCN QT09944524
Contravention date 12 Oct 2024
Contravention time 13:12:00
Contravention location Kingston Road
Penalty amount GBP 130.00
Contravention Entering and stopping in a box junction
Referral date -
Decision Date 17 Feb 2025
Adjudicator Edward Houghton
Appeal decision Appeal allowed
Direction cancel the Penalty Charge Notice.
Reasons Although the vehicle is seen to be briefly stationary within the box junction this of itself is not a contravention. The Council is required to prove that the vehicle had to stop in the junction as a result of the presence of a stationary vehicle. Neither criterion is met in the present case. The vehicle did not have to stop but clearly chose to do so in order to give priority to the - moving – vehicles coming from the right. The Appellant had right of way and could have continued.
The vehicle was clearly not in contravention and the PCN should never have been issued.-------------
Case reference 225014083A
Appellant xxxxxxxx
Authority Royal Borough of Kingston Upon Thames
VRM GJ70JRU
PCN Details
PCN QT10349776
Contravention date 28 Jan 2025
Contravention time 08:15:00
Contravention location Kingston Road
Penalty amount GBP 130.00
Contravention Entering and stopping in a box junction
Referral date -
Decision Date 29 May 2025
Adjudicator Edward Houghton
Appeal decision Appeal allowed
Direction cancel the Penalty Charge Notice.
Reasons I heard this appeal by video link.
The vehicle certainly comes to a halt within the box junction. However this of itself is not a contravention. Under the terms of the regulations (Traffic Signs Regulations and General Directions 2016 Schedule 9 Part 7 Para 11) the Council is required to prove that the vehicle had to stop in the junction as a result of the presence of stationary vehicles. In this case the Appellant self-evidently did not have to stop. He had right of way, but chose to stop out of courtesy to let the vehicles turning right pass ahead of him. In addition, these were of course moving vehicles not stationary vehicles.
The PCN should never have been issued, and the Council shows a regrettable lack of understanding of the law. If similar cases occur in future the question of costs may fall to be considered.