Thank you for that information.
As you have regular contact, a move away from the UK is going to have to be applied for in the family court by your ex wife as without your permission or a court order she cannot simply remove the children from the UK.
If you are concerned that she may try to do remove the children imminently without seeking a court order for permission first (technically this would be child abduction), then you can apply for a prohibited steps order from the family court - preventing the children from being removed from the UK.
If this is more of a longer term threat/ suggestion, then I would suggest you go to mediation first as unless the application is urgent you do have to have attended mediation prior to applying to court.
See here for a link to find your local mediator:
You would apply to court on a c100 form, here:
You send 3 copies of this form to the local family court centre.
Alternatively, you can apply online:
The application costs £215.00, and you can represent yourself so you don't need to engage a lawyer.
I trust that this assists? Can I help further today?