WOW Air has some incredibly low fares to London with availability from January through March 2019. You can get a round-trip flight for just $299. WOW Air is a budget airline and they will charge you extra for lots of things, but this is still a great price for a quick winter getaway.
There’s also $360 round trip flights available to Paris from Newark (EWR).
You’ll have to stop over in Reykjavik (KEF), Iceland on your way to England but for $300, a couple of hours in Iceland definitely seems worth it. That’s also a great country to visit by the way.
Hurry though, this will not last long and some dates are already gone. Make sure to use a card that will earn you some points in the process such as Citi ThankYou Premier (3x TYP) or Chase Sapphire Reserve (3x UR).