This isn't an official NEFTA reply - I just do the scores, I'm not an official.
But.. I did the initial investigation into insurance requirements when we started the NEFTA Hunter series..
Visitors are covered by the clubs insurance, and there doesn't appear to be a limit on the number of visitors now as there once was with the old insurance scheme. So in terms of insurance, so long as you sign in as a visitor (and adhere to the hosting club's rules for being a visitor - so you may need to give an address and sign a declaration that you comply with Section 21 etc and that your gun is under 12fpe and legal) then you'll be covered by the clubs insurance.
The NEFTA Winter league shoots do allow visitors to take part.. so in the absence of an official response... I'd say that you'll be ok to come along and sign in as a visitor.
Steve Hebby or Trev should be able to give you an official answer.