Originally Posted by
DCAorBust
Hopefully someone who has purchased Go Envoy on a UK flight can chime in here with experience, but one other item you probably need to factor into your budget is the tax that the UK levies on business and first class passengers, which I believe applies to departures from UK, not arrivals. As I recall it is on the order of $120 (this would be offset by whatever the coach tax $60ish I think). I'm not certain that it applies to the Go Envoy upgrade scenario, but I can't imagine how they would get around charging the duty.
I've done this at LGW - when they confirm your upgrade (in my case in the club, but possibly also at check-in), you have to pay the increase in APD.
Oddly, when upgrading on UA using a SWU, you are not required to pay the difference. I wonder why that is? Perhaps because the SWU is a non-cash instrument?