If you want to use a city backdrop, try desaturating it or blurring it before use. All you need is the suggestion of their environment; as you suspect, too much detail will distract from the action. Ditto to what kirisute said about camera angle. If Captain Britain is the dominant figure, try placing the camera low, looking up at him. Also experiment with the focal length; a wider angle lens than normal will exaggerate CB's fist in relation to the rest of him.