I had to do this a few months back. There was no service charge for me but that was on a non refundable fare. If yours is refundable there may be a charge, i don't know.

One thing I learnt was that if you are claiming back from insurance BA will be keen to not repay the taxes as they think you will get them back from the insurance claim. I now wish I had claimed my taxes back off BA and then got the rest off the insurance, but thats just a small thing. In view of the insurance claim BA provided a written note within 24 hours which I used to support my claim.