To be clear this is about the rights of the children rather than your rights
The law says that a child is entitled to a relationship with both parents and the courts will enforce this if necessary.
This does NOT mean a 50/50 split of their time - research in Australia has proved that this is not necessarily the best option for a child - but it is unreasonable of her to refuse to allow you to bring the children to your home on alternate weeks even now.
Once you are closer then you could ask to collect them from school on a friday and return them to school on a Monday - and possibly to have them one night in the week on the same basis
Half of all school holidays is also a reasonable request.
As a starting point you need to attend a Mediation Information and Assessment meeting to se if agreement can be reached via mediation.
If not then you will have to apply to the court for an Cild Arrangement Order setting out the time that you spend with the children which can be enforced if necessary
The form you will need is a C100 and the fee is £215
I hope that this is of assistance - please ask if you need further details