You will land at either Terminal B or C. Probably B, but it really doesn't matter too much because you first have to clear immigration, then pick up your bags from the belt and then immediately recheck it as you leave the baggage hall (a fairly obvious recheck luggage desk is on your right). If you land in B you'll take the AirTrain to C. Go through security there. If you land in Terminal C you'll do security shortly after dropping off your bag.
Most of the 767s that operate LHR-EWR are fitted with the new Polaris seats, which are quite nice. I like the odd-numbered window seats. You may get one of the older configurations (2-1-2), so it's up to you whether you like window or aisle.
The LHR lounge is fantastic, one of the best in the UA network. BTW, there are many Polaris reviews in the United forum here in FT.