I've read up on the physics of the matter and now have a sense of what would have happened. The blackhole appeared during PMQs. Starmer eventually realised what was going on, but his attempts to close the hole by threatening to travel back in time and prosecute May for running through a field of wheat did not come close to achieving that, even when he desperately repeated it with a silly voice. Mays attempts to dance only made the power of the blackhole greater.
A number of MPs felt regret that they ditched Bercow, suspecting that he might have been able to shout at the blackhole in a condescending enough manner that the hole might have been put in its place. The regret did not prevent them from being sucked off, fitting that some of them died as they had lived. Others tried variations of routine parliamentary performances but it was not enough, and indeed at one point a dead sheep emerged from the blackhole and savaged some of them.
In the end the nation was only saved because the 14th Doctor Who, Dennis Skinner, happened to be in the public gallery at the time and managed to shout at least six jokes about tory cocaine, sleaze and the royal family in quick succession. And then he picked up The Mace to reveal that it was a sonic screwdriver, which he proceeded to wield with expert skill. He used it to draw a magic line under the blackhole, and then made a joke about the tories snorting it, which tory MPs then enacted. His heroic efforts came too late to save most of the people who were in the chamber that day, but lessons were learned before a line was drawn under the whole affair and we moved on.