Do you have any status in the BA Executive Club i.e. Silver or Gold membership? Those passengers with 'status' tend to be upgraded first if required for operational reasons as they're BA's most loyal flyers.
You may well be offered the opportunity to pay to upgrade either through Manage My Booking on BA.com or at check-in.
I have been upgraded W to J several times from EWR to LHR and occasionally LHR-EWR (very similar route and range of equipment), but as West Coast suggests, I am BA Gold having been BA Silver for a while. This refers mainly to operational upgrades, "involuntary upgrade due to oversale" i.e. lower classes are oversold and people are bumped forward in succession, usually in order of status. There's a chance of economy being oversold at Christmas and this cascade happening but (to the OP) you'd be down the queue if you have no status, and I would expect that flight to be lightly loaded.
On the positive side, at that time of year I would expect an especially low load in Club World so I too would expect a "proactive online upgrade" offer from about a week out which if anything less than about £300 pp is a very good offer. Note that the POUG offer can change up or down in this time, or disappear, depending on uptake. Decide what you can afford and go for it when you see it! Alternatively there may be airport upgrades (AUPs) offered at check-in if you ask which should be a similar kind of price. Either of these options will also give you use of the BA lounge at PHL, which is a touch small/busy but good, and the pre-flight dining if it's a "Sleeper" flight.
"Another Forum" would give a comprehensive record of recent AUP and POUG prices for most routes.
If you're on the 787, all classes are really rather nice with the exception of World Traveller, but Club World especially is above the average.
If you're on the 777, WT and WT+ may or may not be the new generation which is far better than the old generation, Club World is largely the same across the whole BA fleet (with the exception of the LCY A318s and mid-haul ex-BMI A321s).