I'll tell you what will happen:
England and Scotland, and possibly Wales, will wear the armbands and after a lot of huff and puff nothing will happen. FIFA allowed the Republic of Ireland to wear Easter Rising commenorations on their shirts not so long ago, although they weren't World Cup games. I think they'd have difficulty explaining away that one of it came to a showdown
Although FIFA's position is ridiculous, you have to wonder how this situation has arisen - again. Weren't there talks going on about it ages ago? FIFA's posturing on this is well known and the fixtures were arranged months ago