While RSI doesn't have the greatest reputation for having their act together, I can tell you that going through CNAV I was very impressed with their process (and RSI should be part of the same overall system).
I applied for my French pension through CNAV, having already gotten an estimate regarding my pension status some years ago. (Age 56 or 57 IIRC.) A few weeks into the process, I had a letter from the German retirement office saying that they had been alerted to my coming retirement, and "by the way" I was eligible to get my German retirement 4 months earlier than my French retirement, so did I want to backdate it and have the German pension paid starting the earlier date? (Answer: "yes, please.")
OTOH, I still had to make a separate application for my "retraite complementaire" (i.e. the second part of the French pension) through the appropriate website, but once I did that, things progressed quickly and the payments started right on time.
Did you apply directly to Newcastle for your UK pension? Or did you get the request for the form after making your initial application to the RSI? I think the first step in applying for your pension will be for the RSI to send you a listing of what they have on their records about your work experience and to confirm that. They may then ask you to fill in information about your work history outside of France (which is, I assume, what the E205F is all about). As far as I know, they share this information with any other EU countries if you indicate that you have work history and potential pension entitlement there.
Cheers,
Bev