Since there are 10-20 seats open, CO thinks with high probability that they can sell those seats at full price, around $3,000 one-way. As the departure day approaches, and if they predict with high probability that somes seats won't be sold, then they'll release those seats for upgraders.

According what NW/CO agents tell me, most business people buy tickets 1 week to 1 month in advance, paying usually $3,000 each way! ($6,000 is more valuable to them than upgrading a person who paid $300-$1000 RT coach ticket, regardless of status.)

Personal story: (previously posted in CO threads)
I'm an NW Silver Elite member who travelled with my friend to Europe last March. We bought dirt-cheap fares, i.e. non-upgradeable. After I got inspired to upgrade both of us to BF one week prior to departure, I called CO and they told me that upgrade seats were not available on the outbound flight. However, the agent found some on the previous day. So I had the agent change our outbound flight to the previous day. Since I had to pay some money to change my fare to an upgradeable fare basis, CO waived the change fee.

So, for you, keep on checking the IAH-LGW and EWR-LGW flights!

Welcome to FlyerTalk. This is one resourceful board.

- Pat