Modern phones are smart. They’ll update automatically. But I always double-check because my phone once decided I was in Greenland. True story.
If you’re flying from Gatwick, your phone will probably switch mid-air. That’s when you scramble to tell the person next to you, “Oh, we just gained an hour!” They won’t care. They’re asleep.
But if you’re using an old-school watch—like a Casio from 1998—you’ll need to add one hour during summer. Or just leave it. Live dangerously.