That's great that BA have refunded your payment and its really easy to do.
I have attached a link to assist you, however you'll need to scroll down.
When this process is done you may incur a transfer fee, fear not it WILL be refunded to you in 7-10 days. The actual refund takes up to two working days to get back into your bank account.
If you have a credit balance on your account, you can request a refund for your exact credit balance on your account as a money transfer. You can do this through online banking or the mobile app.
Please note, you can only request a money transfer to move the amount of funds you are in credit by and not the amount refunded to you by the merchant.
For example, if you received a refund of £300 yet are in credit of £200 (showing as -£200), please only transfer the £200.
Hope this helps!!
Have a lovely weekend Caz