The decision opens the way to solve a rare but vexing problem; what to do when a genetic disease is caused not by a defective allele in the father's or the mother's nuclear DNA, but in the small amount of DNA in mitochondria. As you may know, mitochondrial DNA is inherited from the mother, and only the mother. Mitochondrial genetic diseases, therefore are always passed from a mother to all of her daughters. There's no genetic mixing, and hence no possibility of avoiding that outcome. To break this pattern requires replacement of the mother's mitochondrial DNA entirely prior to fertilization.
What Britain approved, then, is an IVF procedure in which the nucleus is removed from a donor woman's egg and replaced with the nucleus from the birth mother's egg. That "two-mother" egg is then fertilized with the father's DNA. The child will have nuclear DNA that is half from the mother and half from the father, but mitochondrial DNA that is all from donor. And here's the good news; the mitochondrial defect carried by the child's birth mother is forever removed from all of her descendants, forever.
Yes, it smacks of "playing God", if that's what you wish to call some of the most advanced techniques in medicine. But it's hard to argue against it when the benefits are so clear.