I would like to point out we have a house there, and I have personally lived in Spain for close to a decade!!
You know as well as I do the public healthcare system is absolutely dire, and it’s only the private health system in Spain that is any good, you are making huge assumptions that the child’s family are able to fund that kind of expense.
Secondly,, the economic outlook as well as employment stats means many many young Spanish adults actually come to the U.K. to work. They can’t find jobs in Spain. It’s a massive problem, and the outlook is bleak not buoyant.
The quality of education is much lower, and OP’s dd is likely to fall behind certainly initially. Losing an entire year of schooling to ‘trial’ living somewhere else is really ill advised so close to her GCSEs. The opportunities here vastly outweigh those in Spain - with a much broader scope and choice for post grads. Those looking for apprenticeships and the jobs market generally is much stronger in the U.K.
The standard of living is much lower too generally speaking. It’s much more noticeable in the winter when it’s damp and cold in most of Spain except the very south which can still be chilly. Depending on where they are, it can also be incredibly soulless and boring in the winter, and absolutely baking hot in the summer. It is a much more backward/traditional country, and has never really caught up with Northern Europe as a whole,
Children need their mother most of all, and none more than teenage girls. She is uniquely vulnerable, growing up away from the love, protection and unconditional care of a mother that adores her, it will be a bereavement of sorts to be without a mother at this age for such a prolonged, extended time, it’s too far away for any relationship to exist in the real sense of the word.
Its time her father stepped up and visited dd at home, and for op to spend the money on some wonderful adventures together instead. Paris, New York or the destination choice of the dd, and to start investing in THEIR relationship and not her father’s/in laws that’s for sure! The priority has to be the mother/daiughter relationship for the next few years.