"taxes" and "justification" do not always go well together.

I think we are often used to the idea of taxes being a percentage of the base cost (for the most part). Airline and airport-related taxes aren't always like that.. like the flat-fee of the September 11th security tax and whatnot.. in any case, I suspect that the UK airport taxes are largely flat-fee taxes.. so, the taxes on a cheap NYC-LHR in Y are similar to the taxes on a full-fare F ticket for the same route. Just looking on Expedia, you see the taxes on a Y ticket for that route are $103.63 (AA, JFK-LHR, 7/16-18) and on an F ticket for the same flights, it's $140 or so.

So, for the mileage runner with the low base fare, taxes might seem outrageous (as a percentage of the total cost).. whereas for the higher end, taxes are practically minimal for the person who pays full-fare F.

It's best to think of it from a big-picture sense. Nothing new here, folks.. this is why USA-Canada/Mexico mileage runs are rare.
@:-)