VAT on paintings

I have a client who is an artist and his paintings are displayed in an art gallery in London.

They are sold to the gallery on a sale or return basis and then if a buyer is found a sales invoice is raised to the gallery by my client based on 50% of the final selling price.

Most of the paintings are sold in the UK but quite a lot are sold for large amounts to non-EU buyers. In such cases the gallery (which is a reputable institution) is saying that my client should not charge the gallery VAT on the basis that the painting has been exported.

I am sure the ownership of the painting passes from my client to the gallery and then to the final customer so my client is not the exporter. Or have I missed a trick here?

The gallery...