The Royal Botanical Garden is located out of town in Kew. This is more of a scientific institute than a park, covering over 300 acres. Kew has a huge selection of flowers and shrubs (40,000 species and varieties) and the largest range of orchids in the world. It would be a great place to visit as there's plenty to see.

If you are not sure where to start from you can join a tour which will highlight the main places of the garden.