A parent can organise activities for children as they choose on “their” days. She can go anywhere in the UK and doesn’t have to tell you.
In theory your ex should have your permission to take the children out of the country but if you refused, she could go to court and would be allowed to take them on a regular holiday as long as she had return tickets. Immigration usually just ask that an adult can prove they are related to the child. I take a copy of a birth certificate.
Has she gone away on your weekend without forwarding you?
Thanks for making this post. You may benefit from getting some support from out single parent helpline. They will be able to help you explore your options around this subject. They will be busy as today they are short staffed, but calls will be free.