you can order 3 months before lease end date eg i can order 28 feb for 30 may delivery obviously production times may vary but you keep your current car until new one arrives.
you simply hand your old car to dealer you pick up new car from same day.
there are some issues with car supply to ni due to brexit issues as they require conformity certificates but as to the lack of variety i think thats just down to the fact its a smaller market.