Email them and explain the situation, they may waive it for you. Just explain you want to keep the phone but don't agree that £100 be knocked of the price for such a small chip in the screen.
I think I paid £100 extra and mine was completely smashed.
I've just spoken to the recycle team and they are adamant that I need to pay the £100. I'm annoyed with this to be honest as I've always looked after my phones (case and screen protector) and never saw it when I sent it out.
That's the advice I always give. Preferably with something like a dated newspaper or something else with the date on it as well in the photos if possible
Weird that they want 100. I traded in an s10 5g which has a bigger screen, I marked it as having a broken screen (it has 3 hairline cracks all down it) before shipping and it only knocked 70 off the trade in price