UA can and will sell seats on a flight that is technically oversold (i.e., more seats sold than seating capacity of the equipment), up to authorized capacity. It all depends on historical show factor. In fact, doing so in this situation (last minute, 'walk-up' purchase) is rather lucrative. In other words, the flight status page is not dispositive of whether a flight is in an oversale.

I can't say for certain what happened here, but even under those circumstances, where UA is selling seats, the seatmap can be locked out for airport management inside 24h. I'm sure we've all been on flights that have been oversold in advance, sometimes by as many as double-digits, that go out even, or with empty seats and/or clearing standby pax. LHR, with lots of flights and business travelers frequently making changes to itineraries, might be one market in which United does not anticipate high show factors and will oversell in certain situations.

Seats can also be blocked for airport assignment for other operational reasons, such as broken seats, large groups, cancellations of other flights, etc.