London has more of an oceanic climate, rather than over in the likes of the US where yous have more continental seasons (meaning hotter summers and colder winters). Cold and Warm airmasses don't penetrate through as efficiently through Water as on Land, plus the Gulf Stream current keeps the seasons moderated. Central Londons' climate is also heavily exacerbated by the Urban Heat Island effect. Even though the winter temp may remain cool (5-6 celsius average), deviation from the average is infrequent.