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I assume you are talking about the final disembarkation. When I was on JoS,disemabrkation started around 6:30 til around 8am, although I believe if you want to carry your own luggage off you may be able to get off earlier.
We were on the Jewel a year ago and in Harwich you could get off independently from about 06.15 many people did this to catch the early train for London as the station is adjacent to the quay. If you are travelling independently and have a car at the quayside just fill in your diemarkation form ssaying you want to get off independently and you dont even need to put your cases out the night before. It does have its benefits as you may not have breakfast but at least you can get away before the scramble!!