Hey there anon, I'm on the same night as you I'm guessing.
Unless you feel REALLY strongly about it, I'd just weigh it up against how badly do you want to do king gong, as it isn't the be all and end all, backroom open mics are less hostile and untaped but if your material and delivery's strong you'll last the full five and so will benefit from the hypothetical exposure.
(if you win and blag an open spot, it'd still be taped, if you peformed at any level I think it would be - you aren't signing over copyright or anything though, just the images of that night's peformance)
You can purchase a DVD of your bit afterwards for like a fiver or something, obviously things are taped for security, incident or God forbid, evidence in a libel case. But odds are they won't use it anywhere, and if they did it would be in a positive light (because you ripped, not cos you messed up)
I don't know the ins and outs but if you post this same q up on the manchester comedy forum, or even good ol' chortle many people and the club mcs will know the specifics.
If you're geared up for winning the evening though I honestly wouldn't worry about and lap up the evening, your material'll prob be different in a couple of months or so anyway.
Hope to see ya there, good luck (but not too much, eh?)