WHY HER She's ''Las Vegas''' icy but thawing pit boss.
WHY NOW She doubled her odds in ''Haunted Mansion.''
Thomason is no stranger to the telly in her native Britain, and these days it's hard to miss her Stateside. The 27-year-old has been on U.K. TV since age 14 -- including simultaneous gigs on dramas ''Playing the Field'' and ''Where the Heart Is'' five years ago. These days, she's hitting the U.S. as ice queen Nessa Holt on NBC's ensemble-driven ''Las Vegas'' and playing dual roles in Disney's ''The Haunted Mansion'' as Eddie Murphy's wife and a lovelorn ghost. It was while shooting the Disney flick, in fact, that she won the part of the take-no-prisoners pit boss on the new NBC drama. Despite her workaholic ways, Thomason does crave ''a bit of domesticity'' further down the road...in the form of a Jack Russell terrier: ''I've never had a dog, and the only fish I ever had I killed,'' she says. ''There was a freak heat wave in London. I came back from a weekend away and my fish were boiled. So I hope I don't spoil the dog.''