If you're travelling from the UK it's actually very simple.
Fly with either Continental or Delta, they both route via New York anyway and they will allow stop overs free of charge. This means you could fly to New York, stay there and see the sights for a few nights, then fly from New York to Los Angeles. You could the drive to Las Vegas, it's quite a drive but is worth it in my opinion, probably about 6 hours or 8 hours with a couple of rest breaks, stay there for a few nights the fly back to UK. You could even stop at another US city for a couple of nights, maybe fly down to Houston and then back from Houston to UK, again, there is no cost for a domestic flight from Las Vegas to Houston.
London - New York
New York - Los Angeles
Las Vegas - Houston
Houston - London
If you avoid summer holiday dates between 1st July and 22nd August you should be able to do all those flights for £500 each.