Hello there,
When I did my passport application I had the same question. I filled in just the part for your parents. I was in your same situation. There was no problem for that. As you can see there is also very limited space to fill in....and with all the requested data it would have been impossible there and needed to attach an extra piece of paper.
It worked fine for me. I did the total application before Christmas. I have my British passport now.
I did not fill in any data for granparents.
Hope this helps you.
Bye and good luck!