Promoting some of my comments to a full answer.
Firstly, the EU wide requirement for reclaiming VAT by visitors involves a physical examination of the goods being in the visitors possession at time of reclaimation of the VAT - this is noted in the U.K. Government page on Tax Free shopping (as linked to by Rodney Hawkins):
Show the goods, the completed form and your receipts to customs at the point when you leave the EU (this might not be in the UK).
This serves as a preventative measure against VAT fraud, where someone buys something in the EU for an EU citizen, but reclaims the VAT anyway.
Secondly, claiming VAT on goods exported as freight is also not allowed according to the above page, which is specifically what you are asking about:
You can’t get a VAT refund for:
- goods to be exported as freight