open the campmap\startpos folder. You'll notice several txt files.
Open the file you want to change. (early high and/or late, I guess)
Scroll down to the setneighbours section (about 1/5 starting from the top)
search the line for wessex and remove ID_FLANDERS, so it goes from:
SetNeighbours:: ID_WESSEX ID_MERCIA ID_WALES ID_FLANDERS ID_ENGLISH_CHANNEL ID_IRISH_SEA
to:
SetNeighbours:: ID_WESSEX ID_MERCIA ID_WALES ID_ENGLISH_CHANNEL ID_IRISH_SEA
do the same for flanders, so from:
SetNeighbours:: ID_FLANDERS ID_WESSEX ID_NORMANDY ID_ILE_DE_FRANCE ID_CHAMPAGNE ID_LORRAINE ID_FREISLAND ID_ENGLISH_CHANNEL
to:
SetNeighbours:: ID_FLANDERS ID_NORMANDY ID_ILE_DE_FRANCE ID_CHAMPAGNE ID_LORRAINE ID_FREISLAND ID_ENGLISH_CHANNEL

scroll further down to the setborderinfo section (immediately beneath the setneighbours section) and remove the border between wessex and flanders (and vice versa). The easiest way to do that is to place // before the lines you want to remove (everything after a // is treated as a comment, not as code).

I hope it helps and make sure you made some backup files you want to mod.