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.