From Refunds:
Whether you use the online process or contact TfL Customer Services,
we can credit your online Oyster account if:
- you only had pay as you go credit on your card
- you have five days or less on the tickets on your Oyster card
We will charge a £5 administration fee when processing your refund. In
some cases this may mean that no refund is payable."
So online seems possible but any refund even smaller than you were
hoping. Also, the above may not be much use to you if "credit your
online Oyster account" does not suit.
So for "cash" on line does not seem an option, there are
others
(by post, at a Tube station) but the least impractical may be:
"When you call Customer Services, on 0343 222 1234 (TfL call charges),
you'll need to confirm that you are the registered owner of the Oyster
card you're calling about. You must provide the following details:
- Your Oyster card number
- The answer to the security question you selected when you registered or protected your card online
If you don't have this information, Customer Services will ask you
some other questions to find your card and identify you as the
registered owner.
We can refund you by:
- Cheque (cheque refunds in pound sterling only)
- Payment to your bank account (UK bank accounts only)
- Credit to your Oyster card. We can refund up to £50 back to your Oyster card as pay as you go credit
- Web account credit. We can give you a web account credit which you can use when you next top up or buy a Travelcard using your Oyster
online account"
An "0343" number from overseas might also eat into any net refund.
Overall, probably best to forget it!