Unofficially, in halls of residences, and you're staying within campus, whilst you legally need one, they can't really and won't come into your halls and randomly check whether or not you are watching TV illegally. Simply put it is very difficult to enforce. That said get one if you can, I won't advise you to break the law.
You can find more details and information at their website here.
As for things like 4oD and BBC iPlayer you don't need a TV licence because it isn't a live broadcast. To be honest I've been at uni for 4 weeks and I've barely watched any TV, what I have was on BBC iPlayer and from the TV in our common room. I watch DVDs mostly, I brought lots from home.
And if you must know, you can get channel 4 catch up thing on your laptop!