Fashion is big in London but vibrantly eclectic so you're unlikely to stand out, whatever your sartorial choice.
Many top-end restaurants, bars and clubs will insist on smart attire, especially for shoes, so avoid trainers (sneakers) and flip-flops. Style is pretty relaxed elsewhere, although Londoners usually make an effort in the evenings, whether they’re kitted out in cool T-shirts or more formal gear.
The weather has a mind of its own, regardless of the season: always carry an umbrella or a jacket that can repel a shower or two. And do wrap up warm in winter: the wind can generate horizontal rain (and occasionally horizontal snow or sleet).