Chelsea Physic Garden
Garden founded by apothecaries in the 17th century to the medicinal properties of plants. It was only opened to the public in the 1980s, when it became a charity. The heat-sink caused by its thick walls, combined with the general waste heat of London itself, keeps the garden much

Garden founded by apothecaries in the 17th century to the medicinal properties of plants. It was only opened to the public in the 1980s, when it became a charity. The heat-sink caused by its thick walls, combined with the general waste heat of London itself, keeps the garden much
Check the official site before travelling; hours and prices can change.
More parks

Hampstead Heath
320 hectares of wild north London — swim the ponds in summer, climb Parliament Hill for the skyline.

Greenwich Park
Royal park with the Observatory at the top — best skyline view east of central.

Regent's Park
Queen Mary's rose garden in June. Primrose Hill at sunset, year-round.
Source: wikivoyage