http://www.veuve-clicquot.com/
thats what i associate with the event. have fun.

I think of men in great clothes drinking single malt scotch.

I would think it would have to be a dirty martini with blue cheese stuffed olives.

Mojitos?

Mint Julip

Pimms Cup is the official drink of polo. It is a gin. There is a drink called polo, that can be made using the gin:
1 oz gin
1 tbsp lemon juice
1 tbsp orange juice

Shake all ingredients with ice, strain into a cocktail glass, and serve.

But these days all types of drinks and food are found at polo matches because people are allowed to bring in food and drink to most matches. We bring tons of stuff and then share with our neighbor and vice versa.

I think it depends on where the Polo match was taking place, as to what I'd imagine was being drunk.

If in South America...maybe a local pale beer or a really dark heady red wine. If I imagine the match in Europe...probably something with gin. I'm not sure this would be traditional in either location, but Polo conjures images of mint based drinks for me (but it could just be the horse reference).

Hopes that's what you were looking for or if not, at least it inspires something useful. Good Luck!