You might find when OLCI opens, that there are more seats available to choose from than currently appear for you to pay for.
This happened to us on a J GLA-LHR-LAS trip last year. Although the upper deck appeared full, when OLCI opened, magically a couple of pairs were free upstairs.
DO make sure you check in online as soon as you possibly can. If you don't get the seats you want, you can still ask for help at the airport.
Have a great trip, and good luck!