While we've seen roundtrip prices like this offered by Norwegian before, we haven't seen the one-way option for $51 listed at this low of a price point. Of course with prices this low, you don't get much from Norwegian other than your flight and a carry-on of up to 22lbs. Checked baggage will incur an additional charge.
However, if you can pack lightly all in one bag (tips on how to do so here), you can get yourself to Europe on the cheapest ticket we've seen in awhile. Flights at this price start in mid-January and daily departures are available through the end of February, with returns available in March 2019. Meal service and advanced seat selection aren't included in this fare.
At times the Norwegian site can be a little wonky and display prices in Euros. So, you'll want to make sure you are on the page offering prices in USD, alternatively, you can search via Kayak or another third-party site for these fantastic fares. Using Kayak, I found seats departing LAX on Wednesday, January 23 and returning from LGW on Wednesday, January 30, 2019.
Have a look at all our flight deals both international and domestic departing Los Angeles (LAX) here.
Need a room across the pond? Here's what's available in London for the January dates mentioned on Hotelwatchdog.