I have flown both products many, many times in the past two years and disagree with Justinlau88. Comparing your food/service experience in this extraordinary days (even CX is serving Y food in F as shown in other posts) is unfair. Cabin staff have also been going through an awful time, so your experience is not representative.
I agree with Middlevels that there two different BA experiences whether you fly on the main deck or the UD. The UD has a F-class atmosphere. Tons of space. Just bring a Pyjama and you feel you are in F class. Windows seats have large storage compartments along the windows; some rows have even extra front space. To summarize my experience (maybe 20 segments) on each airline:

Seats: BA much better on the UD. Wider, good seated and lying down. And a no-brainer if you travel as a couple. Main deck, I still prefer BA J seat. The only J seat that beats it is SQ.

AVOD: CX much better than BA. Large screen, great selection.

Food and wine: Both airlines have gone down. But decent in both. No discernable advantage. Slightly better (less mediocre) wines on CX depending on the day selection.

Service: BA service is excellent. Less smiles and name-calling (“make the pax feel important”) than on CX, but consistent and efficient. Staff is well trained. BA & CX equal.

Lounges: Definite BA advantage. BA lounges in T5 are much better than CX at T3, so is the terminal. QF/BA lounge at HKG is (slightly) nicer than Wing or Pier. But as OW emerald, I can use the great BA F lounge at T5 (not CCR but still great) while CX F section at T3 is just the same as the J section. Can also use the CX F lounge at HKG even on BA.

All in all, an advantage for BA J , especially if you have status and grab an UD seat or travel with companion. On BA, I see no reason to fly F as their hard product is not very superior to their J UD product, and quite inferior to CX F suite. Dont take me wrong, I enjoy flying in J on the CX UD (they have 22 seats compared to BA 20), but prefer BA.