Rooftop cocktail bars / restaurants on a nice sunny day are worth finding in London. I visited the Boundary Rooftop bar in Shoreditch with my colleagues a couple of times early in the summer. The downstairs is the Albion cafe and shop on the corner of Redchurch Street, right behind the station. The views over Shoreditch in the sun are inviting, even if it was a little too warm in the conservatory-area (covered glass). We had cocktails and lunch. This was one of the first warmest days of the year.
Boundary Rooftop bar
I had a peach bellini. The cocktails are not the best I have had in London, and they are a little strong. The previous visit, I shared a pitcher of cocktails with a colleague, and we felt the same way about the cocktails - a bit disappointed.
Steak, fish, lobster, and chicken are some of the mains. The food was pretty decent, but the prices for it are a bit steep. I think that you pay for the views, and it is a fairly prestigious place. When you enter, you are greeted a bit like you would be to check in to a hotel. I felt a little bit under-dressed in my jeans and t-shirt, and my colleagues felt they were as well, but there's no formal dress code as they did let us in and other patrons were very much the same.
I had the chicken, and it was tender enough and the vegetables were nice. I could have had a little more food, though, as it was not quite enough. Most of my colleagues had steak on our first visit, and they said that it was alright but the cost was steep. Service was also slower than we'd like, but it was fairly busy as the weather was nice and it was toward the beginning of summer. Many were out wanted to enjoy the weather, and one can certainly enjoy the weather in this rooftop restaurant/bar.