if you used eBay's Global Shipping you are not responsible for the refund as long as you have tracking proving the item reached the UK hub.
You need to be extremely careful how you handle this as you could be seriously out of pocket if you try to handle this yourself.
Your buyer needs to open a case for not received and you respond with the tracking number proving delivery to the UK Global shipping hub – you should have actually put the tracking in to the transaction at the time of posting.
You cite that according to Global T&C you, the seller, are not responsible for any loss or damage in transit once it has arrived at the Global hub.