It will be harder than previous years to upgrade on any route because this is the first time that both Elites and Prestiges can upgrade from the low fares to Europe.

Flights to/from LHR are usually emptier during the middle of the week. You could also consider the option of going via YYZ as there are 4-5 flights per day from YYZ to LHR so more J seats available. It's usually best to book closer to departure date so you can get a better idea on your chances.

shore9 - it's not 42J; it's 40J on this route.