Yes. There is a 2h15min high-speed train service between Paris and London via the channel tunnel (Eurostar), which is missing in the data. With connections, part of France is reachable from London, and vice versa. I guess the Deutsche Bahn data used to build this isochrone map is focused on continental Europe.

Graph theorists would call this a "critical bridge": if you remove this single Paris/London link, then a lot of indirect connections disappear without it. And the UK appears railway isolated from continental Europe, which it's not.