This is a very good illustration of the difficulty in where the 'significant' line is drawn for refunds on schedule changes. After all, all air travel timetables are not set in stone and do need to adjust over the passage of time as circumstances change.
So, in context, where do BA draw the line? 5 mins? 10? 1 hour? 4? Is it different for sh and lh? For different passengers? How can they be fair, reasonable and consistent? A tricky one.