I was on this cruise in late January/early February 2022. My memory is that we were docked in Tilbury very early, at least by the time we woke up that morning around 6:00 a.m. I had air transport through Viking and used one of their bus transports to Heathrow, so I can't help with the self-disembarking information, but I would think you'd be able get off by 8:00 a.m. as that was the time we were able to leave on our scheduled bus transfer. Our cruise was about 1/3rd full due to Covid, so our experience might have been different than yours will be, but Viking seemed to run disembarkation much more smoothly and much more quickly than other cruise lines I've experienced.
Hope you have a wonderful cruise and are able to see the northern lights! We had fantastic light shows twice on our trip, with the best lights we saw being in Alta.