I always MFU from J to F, but as it is a 4 sector trip ex-CAI it is more for the TATL flights, although the CR is nice on the LHR-CAI sector.

IME, most of the passengers are asleep on the flight back to LHR. The food is ok, rather than exceptional at that time of the day, but there are snacks and a choice.

As G-BOAC mentions, the CAI lounge is woeful (is there a better option that BA could use?). I always try to get to the airport as close to the departure time as possible and spend time in the poor shops instead of rushing to the lounge, which is the reverse of what I do at T5