Note that, in the text, it's stated that Britain's claim to the Eastern Seaboard (south of PEI, anyway) is very, very theoretical....and is last mentioned (chronologically) in a period that can be described as "early days". As of the end of the CUT War (last time we heard what the UK was doing), exactly zero UK subjects lived on the East Coast, outside of PEI.
Things won't start settling down until the populations rebound to something resembling carrying capacity (given the modern agricultural, engineering, and medical knowledge of the Emberverse....that capacity is a lot higher than it was in Mediaeval Times, so populations could climb into late 19th-Century/Early 20th-Century levels). Until then, pretty much everything is "for now", and up for grabs.
Look how long "The Meeting" lasted, before "Montival" was fused together, and claimed the entire Pacific Coast to the Rockies.....despite not actually having anyone in the area (which was the entire point of the Westria Project).
IOW, the Norrheimers could turn their attentions SouthEast, and quickly come to dominate the Mid-Atlantic coast, no matter what the UK did. There could be an expanding local power in the SouthEast that eventually claims the coastline up to the Chesapeake. Or Iowa might decide to nip things in the bud, and assemble a coalition to secure the East Coast, before the UK can successfully colonize the area.
Or the Emir in Dakar might wish to extend the protection of Islam to the benighted peoples of the East Coast .
TL;DR: It's only ~50 years since the Change. A lot of the "permanent" things are actually just transitory, and drawing a straight line trend out to CY 200 is really deceptive.