Who said anything about 804s to Skegness.
The trains will be designed for the London services, because that's where they will be operating for the vast majority of the time, even of they do make it to Skegness. I suspect they'll have a similar seating layout to the GWR 800s actually, but with a much smaller kitchen area.
They will possibly make it to Skegness on a summer Saturday but could run as 5 cars which would solve the problem.