This is our dilemma. JAL have a 'free' stopover in Narita overnight. 18 hours to have a beak at the local area, abeit quickly.
That needs to be balanced off the good reports we see for VS.
18 hrs in Narita? Quickly? You bet!
There is a sizable amount of time required (and money, AFAICT) to get from Narita to Tokyo proper. Sure, you could poke around the area of Narita itself, but I'm assuming this 18 hrs is an overnighter. That's probably a sign for you to get some sleep and what not, practically evaporating your 18 hrs to poke around anyway. Although it is rather noble of JL to 'offer' that overnight to you with the ticket.
VS will take you through HKG, not a bad airport to be in and stretch your legs. If you had status with DJ/VS then you could relax in The Clubhouse in HKG airport, which is open air and looks very nice from the outside (I'm sure just as good on the inside), but if you don't, it's still a nice airport to stretch the legs. I'm sure you didn't come 1/4 around the world to laze in an airport, but I guess you make the call as to whether you want a quick (VS) or slow (JL) approach to LHR.
I don't know about VS Y+ but I flew VS J from LHR-HKG and the food isn't anything to write home about. Definitely ranks as one of the worst meals I've had in a premium class, although I don't have a huge amount of said travel to speak for. I'm prepared to bet that the food on VS isn't as good as that on VA, although I haven't flown VA and I'd expect that to happen anyway. Another forum member here has flown VS and said the food and seat were not bad at all - they were in Y or Y+, can't remember.... I don't know about the food on JL - it seems to have a mixed reputation; so I guess the conclusion is that you probably won't get slop on either airline, but don't get too excited either - as long as it is mildly satisfying, add some salt and/or pepper and don't starve; wash down with copious amounts of alcohol.
Besides, which choice is $1,000 more expensive? If JL is the more expensive one, then I think there's no choice - fly VS. If VS is $1,000 more expensive, we may have a dilemma....