Personally, in January I would historically not consider it a cold spell
till daily maxima fell to around 2-3C or less for more than 3
consecutive days (roughly, in my own mind). Night time frosts might come
into play as well. Perhaps in this clearly warmed period of our UK
climate I should raise that by a degree or two to keep up with the
youngsters and the media.
Dave,S.Essex. 6.2C, Vaguely chilly.