Something is weird here. Week ago there was even post on William’s facebook that they will be on Verva street racing. But now they are not there.
Car is prepared by Orlen, not Williams (even if they didn’t want to provide one of their cars, they could just rent one and repaint it). So Williams didn’t come because Russell didn’t or it was plan all along? As renting F1 car isn’t something you can do in matter of hours.
But this event must have been written down in contract sponsorship. Only reasonable explanation is that there is some performance clause regarding car performance and Orlen decided that Williams is breaking it, maybe doesn’t want to pay and Williams as revenge doesn’t want to spend time/money on the event? Because that Williams will replace Kubica with Latifi was “known” from the beginning of the season. There isn’t really a team for Kubica next season. But even with those two reasons it would be much more prestige to get Williams/Russell to Poland, just for show. What will happen in few months will happen but for now show must go on.