As a long time contributor I am sure you know (but for the folks new to travel) that the purpose of a visit to the Abbey on a Sunday is for worship by the faithful, not for sightseeing, as you mention in your question. That has not changed.
All seats are good seats, they all allow participation in the service.
I worry that the emphasis on a "good" seat will make you rush, and after flying all night you may well be pushed to arrive at the 11:15 on time let alone early.
8:00 touchdown, maybe 9:00 out of the airport if no queues at immigration or luggage reclaim, 10:00 arrival at Westminster tube and then a few minutes walk. If your plane is on time.
Where will you put your luggage?
I hope you have a special experience.
The order of service for that service is at https://www.westminster-abbey.org/order-of-service?id=14391
You'll see reference to the Trussell Trust as the charity receiving part of the Collection. They are a very special charity who help many many people in need of food. There is a tremendous need in this country, and the Trussell Trust are top notch.
You will be arriving on the First Sunday of Advent.