UK -Oyster card refund?

Enclose note from http://www.tfl.gov.uk/tickets/faresandtickets/visitortickets/5187.aspx

As far as I know, there is no admin fees if return within 90 days.

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Can I get a refund on any unused credit when I have finished with my Oyster card for visitors?
If the remaining balance is under £5, you can get a refund at a Tube station ticket office, and they will cancel the card.

If the balance on the card is more than £5, you will need to complete a refund form (available from Tube station ticket offices). The card will be cancelled and you will be sent a refund by cheque.

Alternatively, you can send your Oyster card for visitors to the Oyster helpline to claim a refund.

You can get the remaining balance and the deposit of the Oyster card in the ticket counter in the tube stations. However I don't know where you will leave the UK. If you leave from Heathrow Airport, you can still use your Oyster card, it's cheaper to travel with it. But you need to spare more time waiting for the ticket counter in Heathrow. There are always a lot of travellers waiting.
Remark: go to the airport earlier.