The other thing to mention is that you will frequently see a sign at the check-in desks (especially at outstations) that says "upgrade today for £X / $Y". I'm usually booked in Upper Class anyhow, but the cost is definitely in the range of what I would consider paying.

So to get a free op-up you not only have to be on an oversold flight, but you need to hope that nobody on the plane was tempted by the cash upgrade offer... (plus those who try to upgrade using miles at the airport, another area where VS are more advanced than BA)