From the World Heritage inscription:
This historic landscape garden features elements that illustrate significant periods of the art of gardens from the 18th to the 20th centuries. The gardens house botanic collections (conserved plants, living plants and documents) that have been considerably enriched through the centuries. Since their creation in 1759, the gardens have made a significant and uninterrupted contribution to the study of plant diversity and economic botany.
The Kew Gardens are one of the most underrated attractions in London. I went there on Sunday morning and had a wonderful time. Most of the visitors seemed to be locals, as opposed to the mobs of foreigners you see at the Tower of London or Westminster Abby. It took me a while to get there because of construction with the Underground, but if everything is working the trip shouldn’t be too onerous.
View my complete list of UNESCO World Heritage Sites