As others have said, it's very similar. IIRC correctly I've flown on the 25/12 six times.

Christmas themed food and staff usually a bit more jolly. The only major difference was on a Vegas flight in Y on Christmas, there was more passengers in J than in Y and the crew were looking forward to letting there hair down. drink never stopped and they spent most of the flight sat talking to the Y passengers once services were done.

The best part is traffic driving to the airport in the morning and from mid afternoon (I find that on the midday its quite busy as people drive to others) LHR quieter but less business travellers so different dynamic.