TFL’s French WiFi

So, Transport for London appears to be routing WiFi traffic out to *France* on some underground stations.
I was passing through Marble Arch the other day and Swarm was picking me up as about 8K from L’Arc de Triomphe in France
and Bond Street showed me as in the neighbourhood of Sevres (11K from L’Arc de Triomphe)
Since I was underground, the only method of geolocation was the WiFi endpoint, meaning the traffic was going through to France. But *why* ?