i couldnt comment on what the previous poster said about short term transfer from UK but long term i concur - spanish healthcare is free on payment of social security which is paid by your employer (and stays with you for life even if you only work one day). you will also have it if you register as self-employed but thats very unattractive financially - over 200 euros a month, and it doesnt give you all the benefits. silly, really.
as far as i know the monopoly on health insurance for foreigners is held by Sanitas, a partner of Bupa Intl. It is 60 euros per month for a complete plan (availabe from Bupa Intl or Sanitas directly) that includes pretty much everything you could ever wish for, or 20-40 euros for more trimmed packages (available only from Sanitas directly). All those packages are called Healthplan. The company is very good and very courteous, exceptional quality of healthcare and top level customer service - this last one shoudlnt be taken for granted in Spain