Simonm
At 12 yrs old the workload in high school is high and learning a new language at the same time will be difficult.
IMO (and experience) your daughter will fall behind very quickly and it would be very difficult to catch up. Spanish state schools have strict repeat a year system which means if the child does not make the grade in that year they have to do it all again
Learning Spanish to the level she will require will take a minimum of 6 months with lots of private tuition after school - please don't rely on the school or other expat children. the school they will work with you if you have a plan but it wil be hard during the first 6 months at least and can yo afford any slippage in her education.
Can't comment on teh school you mention
You could start intensive Spanish lessons now whilst in the uK but it will still be tough
Sorry to sound negative but I have seen many fall by the wayside at this age
I would advsie and suggest an international school where she will also learn Spanish. This would be my choice for any child over the age of 8 yrs. Too much risk involved
One final comment - do you speak Spanish? If not helping a child with homework (they get a lot) may be difficult
Good luck in whatever you choose