As a UK driver (left) who has driven in Spain and France (right) I'd say remembering which side of the road to drive on is easy when there are other cars around, less so when it's an empty country lane. Roundabouts are tricky. Also rules around priority can be different which is hard. Then you have to remember the customs can be different, in the UK flashing your lights means "after you" whereas in Italy, it means "I'm going", or so I'm told.