I am a Flying Club Silver member. I want to fly from LHR-NRT - can I use 30000 miles to upgrade at the airport? What is the likelihood of an upgrade this way? And, can I upgrade the return route at LHR?
See the upgrades guide for more in-depth info, but the prognosis is: it's seems unlikely you'll get a upgrade from Economy to UC on the day. And always bear in mind that on the day, it's more about how much availability there is, rather than whether you have the right fare-class.
Have you actually booked the Economy ticket yet?