Personally, I would go with whichever flight leaves later (in this case NRT) to help with jetlag. Having said that, both Tokyo flights leave very early in the day, so jetlag is very inevitable. The HND flight leaves at 1045, so with a 11h45m flgiht time, you'll land at 2230 UK time in Tokyo. So just when you'll be feeling like going to sleep, it'll be 0730 in Tokyo, and you've got a full day to face. If you have the option, take JL or NH, both of which leave at 7pm to HND.

The 787 cabin will likely be newer and less dirty, and there might be some technical advantages to the better cabin design (although personally I've never noticed much difference).

Depending on which part of Toyko you're going to, NRT isn't always too much further timewise. Also with NRT, (unless you'd taxi from HND) you have the benefit of a much nicer train (The Narita Express/NEX) into Tokyo, rather than the busy commuter trains serving HND.