Virgin Boi: When offering an upgrade, especially at such a late stage in the boarding process that all other passengers are seated, it is important to remember the impression this gives to full revenue passengers seated in the business or first class cabin.
How would you feel having paid several hundred / thousand more pounds, only to have someone on a far cheaper ticket sat right next to you.
Actually, if you are going to UG someone, I would have thought that this is the time that would cause least offence to the premium class pax. At a guess, a greater proportion of them are likely to be regular pax than us cattle down the back, and those who fly enough as pax will have seen just about everything at some stage.
All premium class pax know or ought to know that sometimes things will happen that mean that Y pax have to be UG. Tell a premium pax: "We've got a bit of a last minute problem with the seats and there's a pax here without one, so we're UGing him to solve the problem and get the flight underway". Would that get a better response than telling the premium pax: "This pax is being UG because he flirted with the check-in girl", or "This pax told the gate agent that he plays golf with the CEO", or one of the other "imaginative" bits of crap that pax come up with every day when trying to blag an UG for free.
I would bet my bottom dollar that the premium pax would be much more understanding of the first reason.