Strange, I have certainly had FIP tickets off DB in Surbiton before. When do you want to travel? Bearing in mind all long-distance services in Spain are compusory reservation it is not possible to book until the trains are released for sale (something like 3 months / 90 days IIRC). As you are going into France as well try
Rail Europe , but I have never tried buying FIP over the phone from them, only in person.
I would not recommend waiting to buy tickets until you get out there as trains can (and often do) get fully booked and, if that is the case, then you will not be able to buy a ticket or even get on the train.
PS. If you only want to travel on local or suburban trains then they are not compulsory reservation (or very comfortable to go long distances in!) and you would be better off getting tickets locally. Did Valladolid to Santander and back to Palencia (300 miles) on a "Cercanias" unit a few years ago for the huge sum of 8 (FIP fare) and got a square backside for my trouble