British Airways has Las Vegas to London on sale for $400 to $450 round trip for travel January to May 2019. The best itineraries for nonstop flights are winter dates. Most itineraries involve BA nonstop to London Gatwick outbound, then a combination of BA and AA flights inbound with either two stops or a long layover like nearly 24 hours in New York JFK. This can be an option for an additional one day destination.
There are also many other intra-Europe connections I saw, like Venice or Edinburgh, but I don’t know if you can turn those airport transits into a near-24 hour overnight transit stop or not. Seems like a good ticket to play around with itinerary options if that kind of international travel might be of interest.
Google Flights is showing low ticket prices in the low $300s for many dates, but I was unable to find any flights price under $400.
British Airways Las Vegas LAS – London Gatwick LGW – Las Vegas LAS nonstop flights
AARP site for $65 off economy tickets pulled up a lowest AARP discount ticket price of $528 for these same dates.
American Airlines Las Vegas LAS – New York JFK – London LGW – Edinburgh EDI – Philadelphia PHL – Las Vegas LAS