I haven't been in the position to request a refund yet, but I saw this on another web forum earlier today. I can't vouch for it personally, but the post has had some replies saying that it had worked:
Edit - beaten to it by a couple of poster!
Ok so whats been happening is that BA have been attempting to direct you to a travel voucher page, there IS a work around, and there's a work around with a couple of extra steps if your flight has already been cancelled. Note that if your flight has already been cancelled you can get a full refund, you dont need to accept a travel voucher, but they keep trying to shepherd you to that page.
This is using google chrome:- From your MMB page where the flight is shown as cancelled go into your browser's settings and disable JavaScript.
- Click on the cancel and refund button.
- Confirm that you now see the correct cash refund form and not the voucher refund form.
- Turn JavaScript back on in your browser settings.
- Click on the "Yes" radio button to select that you are a person in the booking.
- When nothing happens press "Enter" on your keyboard. (This works in Firefox and Chrome)
- The page refreshes and shows an error at the top of the page "email address invalid"
- Click on the "Yes" radio button again and this time it will allow you to select the name of the traveller and complete the rest of the form
Its worth noting that for me that the final step didnt work, so I re-enabled java, refreshed the page, and the form to fill out appeared.
I believe that perhaps steps 1-5 are the only ones required if your flight hasnt been cancelled yet, but beware that they may not give you a full refund in that case. The best advice is to wait until 72 hours before your flight is due to leave to maximise the possibility of a cancellation.