According to FlightRadar24, the 2855 will be operated by G-GATS.
G-GATS is having a busy day. It started off today (1st June) at LGW and operated the BA2878 to Palma de Mallorca and then returned to LGW as the 2879. It's then heading to Faro as the 2854 in order to take you back on the 2855.
Of course, some of this may change during the day.