Hi DD
Where someone sells your works to ultimate customers and he charges you commission ie three parties involved in a transaction, is arguably one of the most complex area of taxation as it requires an agency agreement that is VAT compliance and watertight.
I would suggest to get in touch with a local VAT expert and he will go through the agreements in place and assess the VAT implications and advise on the correct treatment.
If something goes wrong, HMRC always review the commercial agreements in line with what happens on the ground before they can make an assessment.
Hope this helps