The female Blackcap has a reddish brown cap. This page shows photos of the female; photos of the male Blackcap, with jet black cap, are on separate page -- see link below.
Although Blackcaps are present in UK all year round, they aren't resident but are migratory. The problem isn't the cold but the food supply. Birds that winter in the Mediterranean come to UK/north Europe in summer when insects are plentiful. Some birds that spend summer in Scandinavia/Eastern/Central Europe have begun (this millennium) to winter in UK where garden food supplies are plentiful instead of crossing the Alps to go south as previously.