Good luck in the Nottingham Congress.
You don't have to be a member of ECF to have an ECF grade, you just need to have played 10 rated games.
You are free to enter any of the The Open, Major, Intermediate or Minor as you are ungraded. As, it is your first congress it might be advisable to play in the Minor and see how you perform.
It is quite tiring playing 6 games in one day, but they are fun. So, good luck and let us know how you get on