Let make this really simple. Let's forget breaks. Say you had one year group, y10, with lessons at 9,10,11,12... etc
Then another with lessons at 9.30, 10.30, 11.30, 12.30 etc.
Now, in my school, we have about 15 classes per year group. It would not be fair to have one teacher teaching nothing but year 11. This would be really, really stressful. So we are expecting most teachers to teach a variety of year groups. The way timetabling works, you end up with a proper hodgepodge. It would be nigh on impossible to have all your year 10s in the morning, all year 11s in the afternoon, because you have to work around part time workers, long term sick folks, NQTs who can't cope with their groups, all kinds of mishaps that mean your lessons get muddled about.
So, you have 1 lesson at 9. Another at 10. Your next at 11.30, because its a different year group. Then the next one at 1. The next at 2.30. You've lost 1.5 hours of teaching time because you moved from one staggered start time to another.
Now, obviously this is a simplification and an exaggeration. But the principle is correct - you are going to lose a lot of teaching time due to staff moving from one year's timetable to another. And we do not have that slack. We do not have any slack. We are already using non-specialist teachers for most of KS3. We have 1 member of staff that we know is going to go on long term sick as soon as she gets back. I think a lot of schools are in the same position we are.
I don't think this is a lack of "can do" attitude. I think it's just thinking logically about how staggered starts to lessons would actually work. They wouldn't. And if we don't have staggered starts, then we have the crushed corridors I mentioned up thread.
I think we will probably try something like double lessons to solve the problem, but god, double lessons are horrible.