Call queens to ask if they will accept you before you withdraw your current offer, so you can cancel and apply on extra/clearing without risk.
This a perfectly normal sane thing to do. Queens won't think you're greedy by trying to keep a backup and universities have people cancelling offers all the way up to September. Just ring up and tell them your qualifications, they might ask for you to send in a personal statement (I doubt it though, it's in clearing) and some time for a decision. If they say they will accept you, it's a risk free acceptance, if not you can go to Dundee.
I think (Research this yourself before taking my word) the University itself has to cancel your offer from their end for you to be put into clearing/extra, so don't do it from your end. There's a very slim chance they'd refuse to release you, this because you'd be much more likely to drop out and mess up their ratings if you were forced to go there and universities are generally quite nice.