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Easy to say it, but you were a bit unwise entering the box, as the video shows the car in front already stopped when you started to turn right. Essentially you had no exit from the box, and in my opinion would struggle to convince an adjudicator. However wait a bit until others have commented but don't miss the deadline to pay or submit reps on the PCN.

In this case, shall i simply whilst the fine is discounted?

Essentially you had no exit from the box

Not quite true. Could have used the space alongside the stopped car (though that option may not have been clearly visible to the driver). Always have an escape route.

I'd be inclined to pursue the right turn exemption. The offence occurs (or does not) at the moment the vehicle enters the box. I'd argue it reasonable and legal to enter the box as the vehicle in front was at that moment in time stationary while waiting to complete a right turn. The law does not require clairvoyancy, so the action of entering the box cannot retrospectively become unlawful just because the car in front rolled forward to complete it's turn then unexpectedly stopped.

There's also the question of the car in font's indicators (I'd argue it's reasonable to assume the right turn isn't completed if they are still blinking, which we can't tell from the video) and it's subsequent movement - it may have continued to turn right in order to occupy the parking space apparently being vacated in the video. If it did so then it's right turn manouver was still in progress.