I started to look at this but I think if you want any proper comments you are going to have to provide some route details, namely where are you going in Europe, what premium cabins were you booking, did you include a stopover at all in London etc. In the normal course of events the taxes and charges for two one way longhauls should be more or less equal to the taxes and charges as a return. However, the taxes and charges for LON-NYC (for example) would be different to NYC-LON.

Can you provide the routings for when you got the 2-4-1 voucher booking and that for the two one ways please?