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You can certainly delay matters while a court decides if the move is in the best interests of the child or not; and whether the alternative arrangements in place if the move goes ahead are sufficient.
If you believe that she will simply go ahead with the move then you can apply for a Prohibited Steps Order using a form C100 and paying the fee of £215.
It is an urgent applictaion so at this stage you do not have to attend mediation in order to obtain it.
However you do need to attend mediation in order to look at the posisble options.
Much will depend on the reason for the move - but if it is unavoidable then you and your ex will need to look at all the other options - including the child staying with you to go to school if that is feasible - especially if both of the wider families are in the area you currently live in
However in the end the issue will be what the court believes is best for the child.
You can read the latest case law here
Please ask if you need further details