I normally have unlimited data (well several terabytes a month), however when travelling outside the eu this drops to something tiny (200mb). I turn off roaming in those cases, however turn it on when I need things like uber, Citymapper, etc.

In Citymapper I'd like it down download and cache maps when on wifi or when in the uk, but only do the minimum when roaming.

On laptops likewise, sometimes a connection is metered - not just tethered, but wifi on trains/planes, and even some home connections.

Determining when a device is on a metered connection or not is the tricky option, probably inpossible. Perhaps something device manufacturers could incorporate into network settings (a metered/non metreed flag on the connection)