We used Sanitas which is the Spanish BUPA they are much cheaper than the UK BUPA have a look at their website.

http://www.healthplanspain.com/

With regards the scheme you mention paying a small amount a month in a like cooperative arrangement these are not available as yet here in the Tortosa area. See UK website below about this scheme.

If you are not covered for state-run healthcare through other means, the Spanish regional health authorities offer a special pay-in scheme (convenio especial). This is a public health insurance scheme available nation-wide where you pay a monthly fee to access state-run healthcare. The scheme is managed by each autonomous region. Policy holders, including children, pay on an individual basis for access to public healthcare, regardless of pre-existing conditions, anywhere in Spain.

The basic monthly fee is 60€ for the under 65s and 157€ for those aged 65 and above. However, prescriptions are not subsidised at this rate so you would pay 100% of prescription costs. This form of cover doesn’t give holders the right to a Spanish EHIC at this time. The scheme is now available in:

    Valencia
    Murcia
    Galicia
    Castilla y León


https://www.gov.uk/healthcare-in-spain

Changed rules from July 1 2014
What will be changing?

From July 1 2014 you will no longer be able to apply for a residual S1 (formerly residual E106) under which you could currently be entitled to up to 30 months of UK-funded healthcare if you retire early to another EEA country.

http://www.nhs.uk/NHSEngland/Healthcareabroad/movingabroad/Pages/residual-S1-forms-for-early-retirees.aspx

CatSalut, the Catalan Health Service, is the public body that guarantees the right of health care protection for all citizens in Catalonia. Visit their website to find out about health care in Barcelona and Catalonia. Health care is available at the primary health care centres "Centres d'atenció primària" (CAP) Find your CAP centre here Call telephone number 061 for medical emergencies from anywhere in Catalonia. The telephone number 902111444 is an official helpline that can answer any questions you have on general health, illness or health services for residents of Catalonia. If you are a permanent resident then you can apply for your personal Catalan health card called "Tarjeta Santaria Individual" or "TSI" for free health services in Catalunya

http://www.barcelona-metropolitan.com/living/healthcare-in-catalunya/

http://www.telegraph.co.uk/health/expathealth/10628246/NHS-axes-free-health-care-for-expats.html