It's his job to care. Or at least to show up, which he originally wasn't planning on doing at all.
And also, it's just such bad politics. He looks like he doesn't give a shit, and Starmer gets to be the one appearing in Zelensky's video, and with other world leaders. The picture of Biden, Macron, Scholz, and Cameron is not a good look.
I genuinely don't know what his advisors were thinking, especially when they're trying not to lose votes to reform. Are they stupid, or doing it on purpose? Because I cannot think of another reason why they told him this would be fine.