On the face of it, I would say that you should go with BA, due to the non-stops (note, however, that LGW-TPA is not daily). If you or your company are willing to pony up for WT+ (premium economy), then the playing field tilts even further in BA's favor. Otherwise, here are my thoughts:
- I find the Y products to be pretty similar. BA might edge out on crews, generally, but food is a wash, though BA has booze for free, while DL only offers free wine and beer. In terms of cabin comfort, I would say that DL is better, if only because the 2-3-2 on the 767 (or 2-4-2 if you wanted to take the A330 out of LGW) is better than 3-3-3 on the 777.
- For J, I think the carriers are pretty close (assuming you fly the DL 767 out of LHR). The J cabin on the BA 777 feels a bit like a dorm and the layout means you will have to climb over someone or someone will have to climb over you. But the DL seats can feel a bit confined when reclined. Service is comparable and I think DL has slightly better food.
- In terms of the FFP, it is much easier to earn mileage with DL for discount coach. If you fly WT+ or J, then BA comes off a bit better, but the elite bonuses are relatively poor. Also, BA charges fuel surcharges on their awards (DL charges these only on awards originating outside the US). On the plus side for BA are that upgrades from WT+ to J are only 25k miles roundtrip, with solid availability. Also, if you fly in WT+ or J, then BA Silver is not terribly hard to get and provides lounge access to the typically quite nice BA lounges.