Thanks Sphin, Sundance.
FYI, I was worried enough by the ambiguity in the email to chat with FXCM today. The chat representative could not confirm the scope of the email and had to go check. I have subsequently received an email stating "... trading via the fxcm api that does not copy/follow/ is outside the scope of the FCA announcement, the agreement is not needed ...".
It remains unclear to me how they will know that API trading does or does not copy/follow. But I don't see that pursuing this further with them will generate anything clearer.
I hope their statement is correct and things don't fail after 5pm Monday...