Polyball is black tie. There are a very few chaps in lounge suits but they look well out of place. So you'll want a proper ball dress I would think - def to the floor. Strapless optional, spaghetti straps v common. And a throw/wrap. If you're coming from London, it might come as a shock that a lot of people will be dancing properly (a big deficit in my generation's education, maybe Strictly has rectified that now lol)
How do I know? Well, I (along with at least one other EFer) play in the Swan Big Band, and on 1 December we will be playing at Polyball for the 5th year in a row