Based on this: Wi-Fi data collection - Transport for London
(official government web site)
it is not an assumption on @danielr’s part but instead is fact.
The web site makes clear that individuals can be tracked. Presumably the MAC address is hashed or encrypted.
If you are asking … has UK law enforcement / security agency ever used it to track / locate / apprehend someone? That is not known but I guess: if not so far then it will happen.
From that web page:
However, if you would like to opt-out, you can do this by turning off Wi-Fi on your device, turning your device off or putting your device into airplane mode while at our stations.
Translation: There is no ‘opt out’ except to kill your WiFi.
A good thing we are all getting Librem 5 phones then, isn’t it? 