Driving from Gatwick Airport into central London via the Dartford Tunnel, travellers will see two very odd structures at the Dagenham Roundabout. They are known locally as the "Witches' Hats". Some have described them as works of art meant to enrich the lives of those passing by, but these descriptions are fanciful, unsupported, and almost certainly apocryphal. Public artworks in London are commissioned by a local authority have a documented trail along with descriptive signage at their sites.
To my knowledge there are no other structures like this in the London area. And the Dagenham Roundabout does not have any claim to cultural heritage or historical significance that would offer grounds for situating a monument.
Those with access to Google Maps can see a high resolution image of the 'hat' here https://firstname.lastname@example.org,0.1401711,3a,75y,220.02h,83.82t/data=!3m4!1e1!3m2!1sRnGuSwNa85elWZIKHgNyQg!2e0?hl=en
Note that there are no signs, no doors to the structures, or other indications of purpose.
For those without access to Google Maps, here's a static image...
Beyond some fanciful claim to random artwork situated at an implausible location, what are the 'Witch's Hats'?