Hola El Perdido!
Most of us donīt know each other personally, but rather from our postings here on MMīs bulletin board.
Do you think with your English (from London) you would have any problems working or being understood in the USA? I didnīt think so... :-)
This topic has been discussed in other threads on the bulletin board. Aside from the difference in accent, and some vocabulary, you should not have any problem with your American Spanish here in Spain. I have many latin American friends and they have no problem understanding my Castillan Spanish.
With regards to employment here, if you are a native speaker of English, there is quite a high demand for English teachers. It might be hard to have enough classes initially to make ends meet on your own, so make sure you have a little dinero in the bank to be able to survive for a couple of months until you get your feet on the ground. As far as other jobs are concerned, itīs hard to describe your opportunities without knowing your background/experience. I recommend taking a look at a few websites:
www.expatriatecafe.com, www.infojobs.net, www.jobline.es, www.monster.es and
www.computrabajo.com just to name a few. They should give you a good feeling of the job market relative to your skills.
Good Luck!