not much competition for direct flights from London unfortunately.
Can’t really comment on if it’s worth it, as only you will know in the end. But, in terms of the actual product - the WTP cabin whether old or new, is a big plus and certainly a more appealing way to fly, I would suggest. The cabin itself is the draw.
There are the other options as others have already mentioned, by looking at LAX or you could try somewhere like DFW springs to mind.
Just in case you don’t know, although Jan is indeed a 'relatively' cheap time to go to Vegas (the Strip),there is a convention in town called CES in Jan - normally around mid-2nd week in Jan. It’s a busy time which tends to put prices up if you are on holiday, so factor that in to. It’s expensive on the strip as it is! I would choose just before Christmas for best value, in terms of accommodation if on holiday on the strip.