It is a criminal offence for one parent to take a child out of the country without the express written permission of the other parent. If this cannot be obtained then you may seek a consent order from the courts.
If a residency order is already in place then you are normally allowed to take a child on holiday outside this country for up to one month.
It should be noted that if you live in England and Wales, just taking a child to Scotland requires permission.
This is because Scotland does have separate laws. If there is no residency order in place then there is nothing in civil law to prevent or allow a child to be taken out of the country.
If there is a dispute between parents then you should consider a section 8 specific issues order.