In no uncertain terms should you even consider this. A 6am departure, while likely the most "on time" DCA-EWR flight in the schedule, is leaving you with about 1 hour of connection time. That's borderline on a good day where you're on one ticket and UA knows to rebook you on a separate flight. Booking separate tickets means UA is technically under no obligation to rebook you if you miss the 8am EWR-LHR flight. Not to mention you then have to wait most of the day at EWR for the late afternoon departure.

DCA-EWR very often arrives at C71 (I used to fly this route constantly). It's a parking stand with a bus to the terminal. Then the LHR flight could leave from anywhere in "C" leaving you with a short 5 minute walk or a longer 20 minute walk.

My advice: look elsewhere or book DCA-EWR with an overnight at an airport hotel the night before the LHR flight.

-RM