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Afternoon, your sons ex wife can apply to come to the U.K. on the basis of the child on parent visa as the child is British she would be required to show she is actively involved in the upbringing of the child to proceed.
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She would need to show that she financially supports her daughter on regular basis such as school and living expenses there is no certain amount amount. When they come to the U.K. there will be access to free school and the NHS.