If you don't mind the travel time then read his review of Kew Gardens and RHS Wisley as well (they are both further away from London, but are certainly more spectacular). If you'd like to walk around somewhere it bit more atmospheric (but with lots more weeds and nettles!) then how about a tour of Highgate Cemetery? Read Craig's review of Highgate to see what it's like inside -- he thinks it's one of the most beautiful places around, despite its lack of flowers.
Admin – “They describe it as "not strenuous"... but you should be prepared for two hours of walking and being on your feet, which can be quite tiring. There will be lots of stops along the way, of course, so the guide can talk about the locations… more”