As a curious data point, when I bought a new phone and then signed up for a new SIM-only plan with a UK provider not so long ago, they made a big deal about how I now had access to free WiFi on the Underground. This was not mentioned at all during the sign-up process and not something I requested or opted into.
TfL do say they won't identify individuals, but for the purposes of data protection law anything that could be used to identify a specific individual is in scope, so these kinds of systems (and similar ones that have been used in places like shopping centres for a while) might be skating on thin ice if there are also, for example, sufficient CCTV cameras around and recording for an individual to be identified from those and then matched against their phone.
It looks like TfL have been cagey about exactly what precautions are being taken here, and they certainly have other mechanisms that could potentially be used to identify individuals such as CCTV and data from payments and entry/exit barriers, so given the scale of the Underground network and the number of people likely to be affected, it wouldn't surprise me if these kinds of stealthy phone-tracking systems started to come under greater regulatory scrutiny before long.
TfL do say they won't identify individuals, but for the purposes of data protection law anything that could be used to identify a specific individual is in scope, so these kinds of systems (and similar ones that have been used in places like shopping centres for a while) might be skating on thin ice if there are also, for example, sufficient CCTV cameras around and recording for an individual to be identified from those and then matched against their phone.
It looks like TfL have been cagey about exactly what precautions are being taken here, and they certainly have other mechanisms that could potentially be used to identify individuals such as CCTV and data from payments and entry/exit barriers, so given the scale of the Underground network and the number of people likely to be affected, it wouldn't surprise me if these kinds of stealthy phone-tracking systems started to come under greater regulatory scrutiny before long.