We did this when flying to San Francisco from LHR. I just asked one of the guys who was directing people to the check-in desks. It was £130 each IIRC, which was less than the web price. I had to go and pay the extra at another desk, but it was simple enough.
We have upgraded once each way. It was cheaper in the US flying back. They charged us $199 each, which was a bargain. I can't remember the price we paid from LGW to Orlando, but seem to remember it wasn't much of a saving.