Regarding the fare calculation, your ticket will have been priced as the sum of two fare components: the outbound leg in CW as one component, and the return leg in WTP as the other. Your refund should be based on the return leg fare component. The BA e-ticket very unhelpfully does not show any of this information (only a total amount), but if you have access to the GGL line you should be able to ask for this information (at the very last ask for the fare codes for each leg - from that someone on here can probably look up the fare, or we can try and reproduce the fare using the ITA Matrix).

Taxes and fees are another question - as these change from WTP to WT, I would argue they should be included in the compensation calculation, but BA will most likely start from the contrary viewpoint. Better here to read up on the link provided above by CWS.