The Circle line is famously not a circle, indeed has never been vaguely circular in shape. So where in London are the largest genuine geometric circles? I've been to investigate four whoppers, and you can decide for yourself which is the biggest.
The largest circular object in London might be the Millennium Dome, or as it's known these days The O2. It was built for the Millennium, obviously, and thrown up on a former gasworks in two years flat. Technically it's not a dome, more a tent (but then technically it's not an oxygen molecule either, but this hasn't stopped the branding rights being sold for £185m). The teflon roof is 52m high at its loftiest point, representing the number of weeks in a year, and is supported by 12 steel masts (representing months). Officially it's 365m in diameter, to match the number of days in a year, but I own a 200-page coffee-table book about the construction of the Dome which states it's really 320m. Mike Davies' original plan was for the covered area to be 400m across, but this would have overhung the edge of the site and "got in the way of some existing riverbank support structure" so the dimensions were scaled back.
Looking down on the Dome today, its perimeter is anything but circular. The edge of the roof loops down to the ground every thirty degrees, and extra semicircular indentations allow for ingress and egress. But underneath the canopy is a glass wall which curves appropriately, and I suspect this is the basis of that 320m measurement. It isn't possible to walk around the edge of the Millennium Dome, both for security reasons and because the owners use the backlot for storage. A perfectly round (but inaccessible) service road loops the perimeter with a massive diameter of 380m, which is also the length of the "Up at the O2" walkway. By rights this service road should be the largest circle in London, but alas has a physical break at the front where the public walks in so I don't think it counts. Try 320m instead, probably.
We now jump to New Southgate, technically Brunswick Park, towards the northern end of the Piccadilly line. The Great Northern London Cemetery was opened in the 1850s as a joint venture between Colney Hatch Lunatic Asylum and a railway company. They recognised the potential of an out-of-London site for the burial of bodies transported by train, in this case from King's Cross, much as the better known Necropolis Railway ran from Waterloo to Surrey. Direct rail services stopped in 1873, after which the cemetery continued more normally, and burials are still conducted today. I unintentionally walked into a group of grieving policemen by the chapel while I was trying to keep out of the way of a fox.
New Southgate Cemetery has very a pleasant setting on land sloping down to the Pymmes Brook. It also has an unusual spoke and wheel layout courtesy of the architect Alexander Spurr. A chapel sits at the centre, then ten roads radiate out towards an outer rim, a bit like the segments of an orange. Somewhere in the midst of these sectors lies Ross McWhirter of Guinness Book of Records fame. But the most notable burial is that of Shoghi Effendi, Guardian of the Baháʼí Faith, whose resting place is contained within a special locked garden overlooked by an eagle on a marble column.
I walked the ring road, with its 350m diameter, in the hope I was following the largest circle in London. Pictures of smiling Italian grandparents adorned the gravestones in the first sector, while the Arthurs and Marys of days gone by filled much of the centre. The lower slopes were busy with relatives arriving by car, particularly to the newish Greek Orthodox section which spreads down to the river. It was here that the otherwise circular road demeaned itself by including a 100m straight. I was willing to overlook this short aberration as a modern upgrade, but at the end of my circuit the road stopped short of the very last 'slice' and linked to the entrance instead, breaking the ring entirely. A lovely walk, but not quite a circuit.
This one is unarguably a circle because that's how John Nash designed it. He refashioned Marylebone Park into a landscape suitable for the Prince Regent's summer palace, although that was never built and neither were most of the 56 luxury villas Nash planned to include to help fund the project. But the central looping carriage drive survived, known as the Inner Circle, with considerably more open space than originally intended. It's about 330m across, which by coincidence makes it one kilometre all the way round, so the broad lightly-traffic circuit is popular with cyclists. One serious-looking bloke in Union Jack lycra lapped me several times while I was walking round.
The Royal Botanic Society took over the centre of the Inner Circle and laid it out with lawns and a lake. The public weren't allowed in until the 1930s, when the formal rose beds of Queen Mary's Gardens and the famous Open Air Theatre were added. Outside the Circle are allotments, tennis courts and two of Nash's original Regency villas. St John's Lodge is now owned by Brunei's royal family and The Holme by the Saudis, because London addresses don't get much more prestigious. A third villa has been replaced by the campus of Regent's University London, much beloved by international students. It's hard to tell if the waistcoated chauffeurs parked outside are waiting for wealthy students or a crown prince. Very much not your average street, but very much a circle.
Finally to the southwest suburbs of Croydon and an interwar housing estate in Waddon, close to Purley Way. A lot of housing estates include semi-circular roads and some have proper circles, but I haven't spotted any others in the capital on quite this scale. Here are two concentric circles, the innermost with a diameter of about 150m, the outermost double that. Denning Avenue cuts through both, off-centre. The middle of the ring was once an old chalk pit, which perhaps explains why the estate's designers preferred to loop houses around the outside and left the middle as mostly grass.
The outer ring is actually two roads, Crowley Crescent and Cosedge Crescent, both named after Croydon soldiers killed in the First World War. Each is lined with solid but unremarkable semis, faced in plaster and liberally sprinkled with 'No Junk Mail' stickers. Some gardens are hedged-off gravel, others scattered with garden-centre-sourced statues of small dogs, but most are simply somewhere to park. It's not possible to drive round the circuit because both crescents are one-way streets, but to walk it takes just over ten minutes. I did this, and sadly discovered that the very eastern side of the loop is actually a separate straight road, a bit like a soap bubble clinging to a dunked blower. So maybe this circle doesn't officially count either, which'd leave Regent's Park's Inner Circle as the clear winner.
• Caesar's Camp, Wimbledon Common (not circular, but pretty close for an Iron Agehillfort) - diameter 300m [map]
• The new Wembley Stadium is "almost round", but not a circle (thanks Ian!) - diameter 300m [map]
• Roundabout at M25 Junction 29 (intersection with the A127 on the outskirts of Upminster) - diameter 230m [map]
• Valence Circus, Becontree Estate, Dagenham (thanks Lynn!) - diameter 220m [map]
• Some other enormous circle I've entirely overlooked (n.b. needs to be an actual circle, and enormous)