One of the must do things in London is a visit to the Sky Garden at 20 Fenchurch St (aka the Walkie Talkie). The garden covers the top 3 floors of the building and has 2 restaurants and a bar. It's free to go but you do have to book if you want to go up during the day. As you can imagine it is particularly popular at sunset. Getting a breakfast booking in the Brasserie isn't so difficult and well worth it. Although you do pay a premium for the view. 

Hint: It's a little know that if you make the effort and get up early then you can go up to the bar without a booking. That is as long as you get there before 10 am (9am to be safe).  So during the week in winter you can see the sunrise at 8am without having to get up too early and the bar does sell pastries and coffee/tea/hot chocolate to keep you warm. In summer there is a walk-in policy in effect after 6pm on weekdays and after 9pm on weekends, you should be able to see the sunset then.

It is well worth the effort as this is the vista that will greet you.