your company was the owner of the work of art, rather than being an agent/marketplace arranging the sale on behalf of the actual owner, and
ownership has transferred to the buyer while the item was in Germany, and
the value exceeds £135.
If so, I think the first question is whether you should charge German VAT – if you’re either already registered for German VAT, or should be as a result of this and/or other transactions. With the goods being sold in Germany, you would expect the transaction to be subject to German VAT unless you hold whatever evidence of export their tax authorities require.
I don’t see that you would be required to charge UK VAT – unless ownership of the item doesn’t pass to the buyer until it arrives in the UK.
Regarding any amount of VAT/duty payable by the buyer on import, this would be between them and HMRC.