Answered by The Penny Pincher
I don’t blame you for wanting an upgrade, London to Hong Kong, is a long flight in coach class.
Since you have a reduced economy ticket you will have to wait until the day of your flight to request an upgrade. Wait until you pass through security to approach the gate agent in charge of your flight and then inquire about the availability of business/first class seats. Ask the agent what would be required to upgrade. After posing your question be quiet and wait for the agent to give a response.
Also offer to write a nice letter to their supervisor.