custody doesn't go to one parent over the other based on anything more than the best interests of the child. i would be contacting the family relationships advice line NOW and trying to arrange mediation and getting something in writing as to your custodial arrangements BEFORE he moves. get it put in place that the girls are to continue at their current school if either of you move form the area etc

there is no leaning towards mum having custody anymore - it's what is in the best interest of the children so get it in writing NOW