Kew Gardens is really quite a long way away from Finchley. The Tube planning page reckons over an hour travel with multiple changes. I recommend Hampstead Heath, particularly Golders Hill Park which is the section of it right in the neighbourhood. The Heath has a mix of formal gardens and managed natural landscape; it focuses very much on native plants and ecologies, whereas Kew is all about collecting and showcasing exotic (from the point of view of Britain) plants.