Liz recommends SkyScanner. It's a good site, which I've often recommended myself, but it doesn't always find the cheapest flights on specific routes. (It provides a useful starting point, but no more).
Norman suggests Virgin Atlantic but their website does not list flights on this route.
The Manchester Airport website states that the only direct scheduled flights to Las Vegas are with BMI. (My Travel operate charter flights on this route):
http://www.manchesterairport.co.uk/app/Destina tions3?LETTER=L
The BMI website confirms that they operate on this route:
http://book.flybmi.com/
A bit of exploration of the My Travel website seems to show that their flights, from Manchester to Las Vegas, only operate twice a week (Sundays & Wednesdays) and only during the summer months. You can only book return flights and the return date has to be either 7 or 14 days after outbound travel. However, if those restrictions are acceptable to you, their flights appear to be much cheaper than those of BMI:
http://www.mytravel.com/AniteNextPage.asp?p=FL IGHTONLY&s=323684651
Chris