Thanks to everyone for your advice. Here's how it played out: Morning of flight it was about 2/3 full, but E- was totally booked. Daughter had to checkin at the airport, no OLCI, I'm guessing because she needed to show passport, docs, etc. Arrived 4 hours early given the chaos at UK airports lately. However, still no seat assignment at in-person checkin. Several hours later when checking UA app she found she had been assigned 44K: last row, aisle, with bathroom directly behind her. Not wanting to earn her keep by spending the flight distributing hand towels to every patron, she hustled up to gate agent once the desk was open and politely asked if she could get a window seat. She was moved to 35L in E+ at no charge. Just checked seat map and nearly all of economy is full, just a few seats each in E+ and E-. So all in all, everything worked out in the end, greatly appreciate your responses. Hoping someone else might benefit from this information down the road.