This will need to be a phone call - there really isn't anywhere to write to I'm afraid.
If the taxes are indeed a percentage of the fare then they shouldn't apply - you're going to need to call, get someone to read the tax info, then send your booking(s) off to be reissued with residual refunds.
Although it seems shady, it's simply that BA hasn't realised that those taxes don't apply on zero fares and has automatically collected them based on the published fare they used to calculate the other fees and taxes. This issues affects other countries too, like Costa Rica.
You just have to speak to someone, get them to understand and agree with you, and you'll get your money back.