£4.16 charge on bill for o2 travel

[Wemorwell]

I appear to have a charge of £4.16 under things you've bought.
It shows it as o2 travel.
Can you clarify what this is, as on the 25 March that it is showing, I was in Edinburgh, and then Dublin (part of the EU).

[OnePlus]

I have a OnePlus 6 which is normally really good :-). Someone mentioned that even if you had data roaming off, if the wifi signal is week during for example a Skype call, it automatically switched to 4G. Not sure if this is the case, but if it is, that could be the problem. Have you heard of this?

[MI5]

You have the option in settings to use mobile data if the WiFi is poor but it will only do this if data is on. The phone won't override the on / off setting.