I have flown both United and BA between Washington DC and LHR. I am also a United frequent flyer.
In my view, United is OK, and is not bad. But BA specifically, and many other non-US carriers, will have nicer service and food, even in economy. You have to decide if it is worth it.
Hoping for an empty seat next to you? The United flight will likely be full. I have seen more empty seats on BA.