I know a lot of posters hate LHR but I would be cautious about an EWR connection. Most of those flights are domestic aircraft so you would be flying a 5-6 hour leg in domestic “first” class. United’s international configured 757s are slated for the JFK PS routes. Even if they are supposed to roster an international aircraft on the EWR transcontinental route it could be changed or, if you mis-connect, you will likely find yourself on a domestic configuration aircraft. Keep in mind it will be winter and the US air traffic system frequently breaks down with the slightest sign of bad weather. This greatly increases the risk you might mis connect. I have been downgraded more than once on US carriers on the domestic portion of a flight just to get to my destination. If you do decide to change in the US, JFK is the only airport with a lot of international configuration aircraft on the transcontinental routes.
You might consider KLM through Amsterdam. And, if you do fly a domestic leg, I would fly in and out of LA which has fewer weather delays in winter. If you fly internationally direct to California, SFO is fine since the international flights have priority in weather delays. I also find fewer immigration delays at SFO compared to LAX.