They will just sit on you knee, we went to Spain a few weeks ago x
We got a doona second hand off Facebook just for our own peace of mind when travelling on holidays
We’re going next month also and don’t plan on taking a car seat
Legally I don’t think you have to for coach or taxi transfers. If you’re going in a taxi you might be able to request one (or could take one with you) but depends what you’re comfortable with x