you can see here the history of aircraft that have flown this route over the last month
Route Tracker | The BA Source
This can give you an idea how likely you are to get on old/new/refurbed.
Also looking at the seatmap for the day of your flight will give an indication. If I remember correctly WT+ on the older 747 interiors are is between CW and F. on the refurbs its behind CW