Apart from all the fantastic London Parks you can get inspiration at the Chelsea Physics Garden. They are at the moment creating a new part, the Garden of Edible and Useful Plants, which will open on 23 May 2012.
And since this is a big jungle of a city, you could even join the garden guerrillas at night, or - and this is something for the coming weekend - go all tropical at Kew Garden (till 4.3.12)
But if you want to take it one step further, check out the Garden Museum on the South Bank. I recently stumbled across it and need to check it out as soon as it gets a bit warmer. In the meantime I will stick to the warm Green House at Kew.
Have a lovely Weekend!