It's scheduled to be a 747-400, in the low yield configuration. That is to say
this one. Late changes are not uncommon, so you shouldn't take as absolute gospel until you have seen the aircraft on the day.
The information about aircraft type is available in lots of places, starting with BA's own website - look at the timetable pages. To see the scheduled seating configuration, one way is to make a dummy booking on an online travel agency and take it as far as the seat maps before cancelling.