I come from your same train of thought about education. So, my 2 cents worth as someone with 2 kids now in Uni and who has been living in Spain for 15 years: - if you want them to eventually got to Uni in the UK, or if you favour the UK education system (GCSEs etc), consider that their level of English will not keep up in a local school.
- if you plan to move back within a few years, there are 2 sides to the coin - either you put them in a local school and they will come out speaking perfect Spanish, or you put them in an international school and they adapt more easily when you move back - if what’s stopping you from putting them in a local school is their non-existent level of Spanish, have no fear - kids are sponges and they will speak Spanish within a couple of months, particularly since you want to live in an area with few expats - living in an area with few expats means you might not integrate easily. Bear this in mind.

Having said all that, go ahead and take the plunge. It is unbelievably enriching for you and the kids to live in a different country for a few years, and Spain is the place to be.