But is there anything practical or in the United T&Cs that would stop me from instead booking a very long EWR-LHR/LHR-EWR trip and another very long LHR-EWR/EWR-LHR trip nestled inside it? Thereby saving about $2500 per trip. Am I missing something obvious?
This is called back-to-back ticketing and is expressly prohibited by section 6(J)3 of the
Contract of Carriage. How much they'll care, and what they'll try to do about it, tends to depend upon the severity of the situation and the frequency with which someone attempts it.