If I cast to a hotel TV in France from my phone which is VPN'd to my home in the UK, will the TV retrieve the content via the hotel WiFi that it's connected to, or directly from my phone?

Hope the title makes sense. But there may be some things I’ll want to watch that I’ll need to be on my home UK IP address to access.

Unless you are screen casting then the TV will use the hotel wifi.

Casting is basically telling the unit to download a stream.

As the other comments said, the way Google Cast works is by telling the player to play some remote URL. Usually this would mean that the player itself needs to have access to the media directly. There are some situations where an app might host a local server for the receiver to stream from (e.g. LocalCast), but usually that’s not the case.

Connect your own Chromecast to the TV and have it VPN to home. That’s what we do when we travel. We don’t use the TV’s software, we just use it’s HDMI port.

Main reason for us is to reduce latency when playing Plex content (one hop instead of 10-20+ hops).

Same situation I think. The Chromecast dongle pulls the media directly from the hotel wifi…unless your chromecast supports setting up a vpn on the chromecast itself? Mine is an older model so I have no access to it directly without the Home app on my phone.

Chromecast with Google TV can setup a VPN client on the unit directly.

Startup, use your remote to connect it to wifi, start VPN from the network menu, start your media/streaming app.

I use WireGuard on mine.

Well currently I’m only using the old dongle without google tv and I’m currently staying in a place that uses a captive portal with their wifi network so…I can’t even use my chromecast, it won’t connect to the wifi because I have no way to enter the captive portal password.

Consider upgrading. Newer models are so much better.