Who owns the goods when they re-enter the UK?
If you still own the goods at this time then your supply to the end customer takes place in the UK and there should be no need to register for VAT in the Netherlands. (Indeed, it is possible that your Netherlands supplier should not charge you VAT).
If the goods are the property of your customer at the time that they re-enter the UK then the Dutch officials are right. The place of supply of the goods is in the Netherlands (subject to the Distance Selling rules) and the supply is subject to Dutch VAT rules. As your supplies are to private individuals then you may be required to account for Dutch VAT on the supply.
The distance selling rules complicate this position so I would strongly encourage you to seek a professional review of your position.
VATease can do this for you at a reasonable rate. Please contact me if you wish to discuss this.
Regards
Paul Taylor
0121 778 4299
http://www.vatease.co.uk/