You are ok due to E.E.C. Council Directive 92/111, part of the Sixth Directive, Article 28N- Transitional measures.
In my experience a copy of the CAA register print out, together with the new US registration document (which should tie in with the de-registration date) is enough evidence to prove the aircraft’s status in this situation.
HMRC no longer issue Free Circulation Certificates as they had no legal standing within the Community, and it is highly unlikely that they will supply any paperwork to confirm the VAT status, other than probably similar comments to mine above.
As I said above I cannot foresee any problems as you have enough evidence to show the aircraft’s history within the EU
(Excerpt from letter from HMRC when we were transferring our aeroplane to the N reg)....