Both London and Berlin are excellent locations to hold the conference and there are many people on both sides arguing for London and Berlin.
In London, December's average LOW temperature is HIGHER then Berlin's Daily Mean. London's average HIGH is almost 2x that of Berlin's (wikipedia Climate charts for the cities)
It is possible for a city to be cold (Berlin) while not being dreary and wet and clammy (London). In any case, good luck with the conference.