You're missing the fact that the system is also divided into explicit signal blocks and those blocks do report train presence to a centralized system[1]. I am not certain if they want to share that information, but if someone got serious about building a system like signalbox I'd expect that NetworkRail would either offer their data, a degraded version of their data on a lag for security reasons, or consider those security reasons so serious that they'd attempt to prohibit deriving that data from cell signals.

1. Mostly, some signal blocks are still entirely manually managed but, IIRC, at this point those manually managed segments are low traffic areas where only one train is allowed into the block group at a time even if the old signal management systems would allow multiple trains.