What do you have to see in London? And what do you do there on a Sunday evening?

Question

My friend and myself is in London just for one day which happens to be a Sunday. What do you have to see? I mean things such as Big Ben, Buckinhgam Palace and so on.

If someone can give us the underground stop for these as well, then great.

I'd like to plan the day well so we may get some time over to hit the shops on Oxford street.

Also, as we're staying on a Sunday night when everything closes early, what should we do in the evening? All I can think of is going to the movies (clubbing not an option here).

Thanks for any help.