I'm no hot drink expert, but I think I can help you here as I drink hot drinks all year 'round.
I'd get some nice tea like PG Tips or Yorkshire Tea. I wouldn't worry about a teapot, just let people use teabags and mugs. Any kind of sugar will do--normal white granulated is fine. Maybe some artificial sweetener for those who use it (I hate the stuff so not sure which is most popular).
For coffee I like fresh, real coffee made in a french press, but when you're talking more than 3 or 4 people it's not really the way to go (only makes a few cups per French press). So in this case, instant would be fine. Just get some nice quality instant, maybe something labeled "fair trade" would be nice.
I'd put out a bunch of little spoons and lots of mugs and people can help themselves. Maybe a dedicated mug for dirty spoons (after people have put their sweetener and milk in and stirred, etc.)
It seems people really go for lowfat milk here (usually has a green lid)--I guess it's a bit healthier, but I prefer whole milk. I guess you could get a container of each. It might look nice if you put the milk in little jugs, but not necessary.
Maybe even get a box of instant hot cocoa packs for those who don't do coffee or tea.
Cross-posted with balmerhon. So you're expected to make the drinks for everyone? That seems awfully akward to me.
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One sugar just means one teaspoon of sugar. Just normal white sugar.