Tricky! Adam Peaty certainly didn't win it on his own but he might still qualify for the purposes of your question anyway!
By the end of Day 8, Team GB has won 27 medals. A day later, the total had risen to 31. So the 30th medal must have been won on Day 9. The four winners that day (but NOT necessarily in chronological order) were Max Whitlock (Gymnastics, Gold), Charlotte Worthington (Cycling, Gold), Declan Brooks (Cycling, Bronze) and the Men's 4 x 100m Swimming Team (which Adam Peaty was a member of) with a Silver.
Source:
https://www.standard.co.uk/sport/sport-olympics/tokyo-olympics-2021-every-team-gb-medal-winner-final-count-b949601.html
While there are lots of websites with the medal results, I've been able to find one that either lists those four medals in order or that provides a detailed schedule (with event timings) for Day 9. Sorry!