Strangely, it appears that, on the basis of one year of college Spanish combined with lots of practice from being the closest thing to a bilingual person at a job I had, I would be better off in Spain than England.

There's a comparison of El Cid in Old Spanish and modern Spanish at wikipedia (see link below), and they don't appear to be very different. Spain was not conquered by non-Spanish speaking foreigners between 1000 and now, so it's not surprising that the language has changed less than English.

http://en.wikipedia.org/wiki/Old_Spanish_language