Ah ok. If it was me, it get them to take the MDF skirting around the corner of the square set opening into the ensuite about 4 or 5 cm, then tile from there.
We had this same dilemma in the laundry and into the mud room as we were only going to have the tiled skirts in the laundry. We couldn't figure out where to change over so we ended up doing the tiled skirts for the mud room as well.
If you have time, can you hit some Simonds, M'con or Carlisle displays? They have tiled skirts as standard, be interesting to see how they execute the change.