You're meant to fill out a declaration at the time of purchase so you wouldn't get charged the VAT in the first place, you can't claim the VAT back from HMRC, you'll need to go back to the supplier and get them to correct it.
http://customs.hmrc.g...ent&id=HMCE_CL_000077
"2.2 Do customers obtain a refund of VAT from Customs?
No. Zero-rating works by the supplier not charging VAT.
If you are charged VAT incorrectly, your supplier may be able to make an adjustment to his VAT records and refund you the VAT. Further details of how this is done are in Notice 700/45 How to correct VAT errors and make adjustments or claims. We do not make refunds directly to customers."