As the goods will be exported from the UK to Spain, you will need to use a UK based freight forwarder.
On import to Spain a declaration will be required. As the goods have been in free circulation in the EU, you may be entitled to Returned Goods Relief.
Further information about exporting goods to the EU and Returned Goods Relief can be found by using the following links:
Export goods from the UK: step by step
EU business: EU Returned Goods Relief