At LHR those customers without assigned seats run the gauntlet on a first come first served basis. Customers are advised at checkin if there is a problem and then directed to a customer service desk. Fare basis, gender, reason for journey are not a part of the consideration. Staff are faced with a list of names for the flight, and we have no way to know who will no show and who will turn up and be really vile. Personally if I am in zone R I will always run through the card holders and ensure that late bookings are seated on oversold flights. Apart from that it is just luck of the draw, and only downgrades can be dealt with at the gate. The emphasis is to keep as much work that is not boarding away from the gate. Passengers with no seat are advised when the flight closes if they have a seat or if they are being accommodated on the next flight with compensation.

The moral, if you do not have pre assigned seats, use OLCi at -24 to minimise the risk.