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HMRC internal manual

VAT Clothing

The design test - is the article designed for young children?: Garments which may be standard-rated irrespective of size

A wedding dress could be within the Notice714 size limits but, since the legal limit for marriage is 16, it would (unless it was a fancy dress costume) be ineligible for zero-rating. You will find examples of similar garments in VCLOTHING9500.