Welcome to Flyertalk and the BA forum, cdb_london, it is good to see you here and I hope you will continue to participate in this board.

It sounds like a facet of a long term glitch in MMB, there are various manifestations of it, here is another example:

http://www.flyertalk.com/forum/briti...ced-error.html

As mentioned in that thread, you can be guided by Baggage Essentials on BA.com for the CW section, and it is indeed the case that UL should accept BA's limits on their sectors, though I don't have personal experience of that. I suspect the other sectors are perhaps causing the incorrect baggage to show.