Well I for one didn't think it would happen. I'm pleased it has. I'm sure the spotters will love it too.
Maybe as more of their 787s come online they will cover either the thin routes, or those which the 757 is used on to provide greater frequency. It could be that there are 757s coming free and that could be what is is being used.
Although I am not a fan of United, I can see why NCL would work for them as feed into their network isn't great at NCL with Star alliance partners, meaning UA are usually the most expensive by far from the NE now that there is no BMI to feed into them at LHR. American and Delta are already served through LHR or AMS/CDG respectively, so UA would make sense. Given that it is non stop, so long as the fares are competitive then I'll be trying this out for sure.