Your Ex can not take your kids out of the jurisdiction of the UK without your consent or court orders, if she tries to take the kids out of the UK, then she would be liable for abduction charges of the kids.
You would have to apply for a prohibited steps order.
This limits parents from carrying out certain events or trips without the express permission of the other.
This is useful if you are worried your ex-partner may attempt to, for example, take the child abroad or to another area of the UK without your knowledge.
Failure to comply with this order is treated as a criminal offence and can result in the person who has taken the child being charged with kidnapping.
You can apply for prohibited step orders by making an application under the Children Act 1989 under section 8.
You can apply online using the below weblink-
For prohibition orders, you can apply for emergency orders. , in order for an urgent application to be successful, there must be strong evidence or an imminent threat. In these circumstances, the application is made ‘without notice’. This means that the respondent would not be aware that the order is being made and nor would they be present at the hearing. If an order is granted on this basis, the Court will list a further ‘return’ hearing when the respondent will be required to attend once they have been served with the application and emergency order made.