First, you need to check whether the fare rules allow refunds. Even they do, assuming that your ticket was a combination of two halves of two return fares, BA will calculate how much it would have cost had you bought a one-way ticket so that in the end there may not be anything to refund. Don't assume that BA now charges one-way fares - they are predominately combined return fares.
However, I think that taxes are always refundable.
You really need to speak to BA about this or quote exact fares (with the route) otherwise - it's just anyone's guess.