November would normally be the start of the winter timetable. Perhaps it's down to that "availability of seats"? They can only guarantee similar flight times/routes up until the end of summer ops and don't want to further muddy the waters?
Does the regulation also state that the airline cannot place a time limit on future dates?
Personally, I think it reasonable to expect most people to have made new plans to travel within the November cut off date and that it's not really that constructive to criticise them for offering a perfectly reasonable time frame to rebook what for most was a weekend getaway gone wrong. I can't imagine BA turning away the relatively few people who try to change to beyond November anyway, but for ease they need to put some kind of restriction on it. Yes it was BAs fault, but they are going to deal with it and pay heavily for it.