I wrote an answer, but it didn’t get published..
Still, something is not clear…
There are inter PLMN Handovers which are possible, and so are inter PLMN cell reselections. If both handovers and reselections are possible, it means that you shouldn’t wait for a total loss of coverage before going to another PLMN… In 3GPP, it says that a loss of coverage is one reason for triggering an interplmn reselection. But if the interplmn neighbor is described in the SI5/SI2 messages, wouldn’t the MS be tempted to do the reselection?
That’s an open point, I’m still looking for the answer..