It is the time of year when we are regularly advising separated parents on the law in relation to foreign travel.
The 'stock answer' we can give as lawyers is: seek consent. It is informative however to go back to the statute and consider why we revert to this advice.
There are a number of relevant domestic and European laws relevant to this issue but particularly for this short piece The Children Act 1989 and The Child Abduction Act 1984 are relevant, as well as case law.
Where both parents have Parental Responsibility (as is usual for a married couple and this article will presume that is the situation) and they agree, a child may leave the jurisdiction of the UK.
If there is a Child Arrangements Order in place then neither parent can remove the child from the jurisdiction without the others consent; s 13 of The Children Act 1989 states:
If a parent has a 'Lives with' Order (previously known as a Residence Order), they may remove a child from the jurisdiction for a period of up to one month. As Lord Justice Thorpe states in W v A  EWCA Civ 1587: 'Thus the obligation on the primary carer with a residence order is either to obtain the consent of every person who has parental responsibility, or apply to the court for permission in relation to any planned removal of more than 28 days’ duration.'
What then, if there is no Child Arrangements Order in place – be that Lives With or Shared Arrangements?
Alongside the Children Act, is the Child Abduction Act 1984. Under the Child Abduction Act it is a criminal offence to remove a child from the jurisdiction. A parent commits an offence of child abduction if s/he takes a child out of the UK without the consent of the other parent or the permission of the Court. This applies whether or not a Lives with Order is in force.
The parent had taken all reasonable steps to communicate to the other parent with parental responsibility the intention to remove the child even if they had been able to actually communicate with them
The other parent with parental responsibility has unreasonably refused to consent.
Therefore whether or not your client has a Lives with Order, it is important to try to obtain the consent of the other parent to a holiday outside the jurisdiction. Having an email exchange recording that agreement is good evidence that the consent was given. It is good practice to provide the other parent with as much information about the trip as possible – flight details, where the parent will be staying with the child/children and contact details whilst they are there, who else will be on the trip, and what arrangements for Skype or telephone calls can be made whilst they are away. Should the case end up in Court, the Court is likely to be more favourable where the client can show that s/he provided this information because it adds weight to an argument that consent by the other person was unreasonably withheld.
If there is no agreement from the other parent regarding the trip, then the parent wanting to travel could make an Application to Court for a Specific Issue Order. Or, the parent opposing the trip may make an Application to the Court for a Prohibited Steps Order to prevent the travel if there is no Child Arrangements Order in place (i.e. no automatic ban in place).