That's GBP200 per person and I don't believe what I'm about to say will eat up all of that GBP1400.
The US carriers don't give certain things for free any more. Meals for one thing - if you want food you pay extra for it. They also charge for baggage. Now, I think that because you are an international passenger you'll get 1 bag per person free for the whole journey and free meals on the transatlantic flights (but check). But you will have to pay for meals on the domestics (if you want them) and if you check more than 7 bags you will pay for the extras. Are you going to want a drink on board? Do Delta charge for drinks? Do you want to watch a movie? Do Delta give you the headset free? Do Delta charge to provide a blanket and pillow (I kid you not) if you want one? Check what other extras you might have to pay for if you go via Detroit or Atlanta, add something for coffee and doughnuts at the change over point and then add that to the cheaper fare.
Now factor in the intangible which is the uncertainty and hassle associated with changing planes and just consider how much you might have to pay for an hotel should you need to spend a night en route. Personally I'd also factor in the (lack of) service on DL compared with BA and VS.
Then consider the consumer protection that is
EUR-Lex - 32004R0261 - EN.
Finally there's the elephant in the room; safety. I'm going to get roasted by some people for this but landing and takeoff are the most dangerous phases of flight. I like flying; I consider flying to be extremely safe; I have flown Delta with confidence. But, I do think about how many take offs and landings I'm going to have to make when I'm booking and I do my best to minimise them and I'm prepared to pay to do that.
Like I said, I doubt any of that adds up to GBP1400 but I believe the *real* difference is quite a lot less than GBP1400.