I found a workaround for cancelled flights to enable you to get a refund even if clicking on refund takes you to the voucher page.
It is similar to the JavaScript workaround already mentioned in relation to Avios bookings but it includes an extra step.
- 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.
Hope this works for others.