Very few direct non stop flights to Vegas from the UK, if Im right isn't it VS 1 x daily from LGW & 2 x weekly from MAN plus the daily BA from LHR.
Could very well work out cheaper to do a flight to SFO or LAX then jump on a $50 Southwest flight to LAS.
I've never flown into LAS but can imagine what it would be like down the back of the plane, party bus atmosphere springs to mind.