This is probably a stupid question but have you unpacked the Kingdoms files? Each campaign has its own pak file which needs to be unpacked separately. The files are all in the same directory as the original vanilla campaign pak file, and I think the Kingdoms disk contains separate unpackers for each. After running it the relevant EDU for Britannia will be in the folder mods/Britannia/data. You then need to delete the two geography files and set up the -io switch, the same as for vanilla.

Hope this helps.