You need to ensure any claim for zero-rating goods sold under
VAT retail export scheme is substantiated by a correctly certified
form
VAT 407, or an approved version or adapted
retail export scheme invoice (VAT refund document). To comply
with EC legislation, the VAT refund document must be certified
at the final point of exit from the EC. (Overseas certification
at the point of destination outside the EC was withdrawn in
January 1992).
Certification of the VAT refund documentation will be at the
final point of exit from the EC, subject to the application of any
of the exceptions outlined in
VATRES4000
No form of overseas stamping is acceptable unless exceptional circumstances exist. You should report any case you cannot deal with to the Retail UoE.