Returns From France To UK

Hi,
Looking for any help with the correct process to get returns back from France to my WH in England. We can ship the order to the customer with no problems at all. But when it comes to returns how do we get this back to England and be able to refund the customer within SLA and avoiding any delays at customs. We have an online returns portal via a third party but doubt I will be allowed to direct customers to this although it would solve all my issues instantly.
Any help from sellers that already return from France to England would be hugely appreciated.
Thanks.