Rose Garden
The roses are mixed with herbaceous plants and bedding flowers so there's always some horticultural interest no matter the time of year, though the roses' peak is in June. Two interesting fountains depict Diana the huntress and a boy seemingly wrestling a dolphin.

The roses are mixed with herbaceous plants and bedding flowers so there's always some horticultural interest no matter the time of year, though the roses' peak is in June. Two interesting fountains depict Diana the huntress and a boy seemingly wrestling a dolphin.
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