The main part of summer days is warm, sometimes hot. Ideal light summer clothes, but in the morning and evening can be quite cold. And I think he warm woolens may be required no earlier than October.
Winter come soon and coldest days of the year, although we can't say that the city suffers from overly severe frosts.
Also in London there is the rainy season. And of course I dream to visit there and see their own eyes the real London rain.