I noticed that the start-up location of Chinese IP addresses is in London.

According to my research, the reason may be that the server of www.windy.com (35.201.88.123, AS396982 Google LLC, Kansas City, Missouri, USA) located in Google Cloud is inaccessible in China [1], so windy switched the resolve in China to b1.windyty.com (139.162.210.148, AS63949 Akamai Technologies, Inc. ,London, England, GB), and the misconfiguration of this server caused the start-up location of all Chinese users to be in London (that is, the geographical location of this server). It can be verified using the curl method [2].
[1] I don't know the reason, but using the traceroute command, there is no ICMP echo packet outside of China, so it is speculated that it is blocked by GFW.
[2] Same IP address, different geoip:
$ torsocks curl https://www.windy.com/ --resolve "www.windy.com:443:35.201.88.123"
[...]<meta name="geoip" content="45.90.185.109,51.2993,9.4910,DE,">[...]
$ torsocks curl https://www.windy.com/ --resolve "www.windy.com:443:139.162.210.148"
[...]<meta name="geoip" content="45.90.185.109,51.5095,-0.0955,GB,London">[...]